WELDER - Bharat Skills · The material is not the purpose of self learning and should be considered as supplementary to class room instruction. TRADE PRACTICAL The trade practical

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NATIONAL INSTRUCTIONALMEDIA INSTITUTE, CHENNAI

Post Box No. 3142, CTI Campus, Guindy, Chennai - 600 032

DIRECTORATE GENERAL OF TRAININGMINISTRY OF SKILL DEVELOPMENT & ENTREPRENEURSHIP

GOVERNMENT OF INDIA

TRADE THEORY

SECTOR: Fabrication

WELDERNSQF LEVEL - 4

2nd Semester

© NIMI, Not to be republished

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Sector : FabricationDuration : 1 - YearTrade : Welder 2nd Semester - Trade Theory - NSQF level 4

Copyright © 2018 National Instructional Media Institute, Chennai

First EditionFirst Reprint : January, 2019 Copies : 5,000

: September, 2018 Copies: 1,000

Rs.140/-

All rights reserved.

No part of this publication can be reproduced or transmitted in any form or by any means, electronic or mechanical, includingphotocopy, recording or any information storage and retrieval system, without permission in writing from the NationalInstructional Media Institute, Chennai.

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Website: www.nimi.gov.in


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FOREWORD

The Government of India has set an ambitious target of imparting skills to 30 crores people, one out of everyfour Indians, by 2020 to help them secure jobs as part of the National Skills Development Policy. IndustrialTraining Institutes (ITIs) play a vital role in this process especially in terms of providing skilled manpower.Keeping this in mind, and for providing the current industry relevant skill training to Trainees, ITI syllabushas been recently updated with the help of Mentor Councils comprising various stakeholders viz. Industries,Entrepreneurs, Academicians and representatives from ITIs.

The National Instructional Media Institute (NIMI), Chennai, has now come up with instructional material tosuit the revised curriculum for Welder, 2nd Semester Trade Theory NSQF Level - 4 in FabricationSector under Semester Pattern. The NSQF Level - 4 Trade Practical will help the trainees to get aninternational equivalency standard where their skill proficiency and competency will be duly recognizedacross the globe and this will also increase the scope of recognition of prior learning. NSQF Level - 4trainees will also get the opportunities to promote life long learning and skill development. I have no doubtthat with NSQF Level - 4 the trainers and trainees of ITIs, and all stakeholders will derive maximum benefitsfrom these Instructional Media Packages IMPs and that NIMI's effort will go a long way in improving thequality of Vocational training in the country.

The Executive Director & Staff of NIMI and members of Media Development Committee deserve appreciationfor their contribution in bringing out this publication.

Jai Hind

RAJESH AGGARWALDirector General/ Addl.Secretary

Ministry of Skill Development & Entrepreneurship,Government of India.

New Delhi - 110 001


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PREFACE

The National Instructional Media Institute (NIMI) was established in 1986 at Chennai by then DirectorateGeneral of Employment and Training (D.G.E & T), Ministry of Labour and Employment, (now under DirectorateGeneral of Training, Ministry of Skill Development and Entrepreneurship) Government of India, with technicalassistance from the Govt. of the Federal Republic of Germany. The prime objective of this institute is todevelop and provide instructional materials for various trades as per the prescribed syllabi (NSQF Level 4)under the Craftsman and Apprenticeship Training Schemes.

The instructional materials are created keeping in mind, the main objective of Vocational Training underNCVT/NAC in India, which is to help an individual to master skills to do a job. The instructional materials aregenerated in the form of Instructional Media Packages (IMPs). An IMP consists of Theory book, Practicalbook, Test and Assignment book, Instructor Guide, Audio Visual Aid (Wall charts and Transparencies) andother support materials.

The trade practical book consists of series of exercises to be completed by the trainees in the workshop.These exercises are designed to ensure that all the skills in the prescribed syllabus are covered. The tradetheory book provides related theoretical knowledge required to enable the trainee to do a job. The test andassignments will enable the instructor to give assignments for the evaluation of the performance of a trainee.The wall charts and transparencies are unique, as they not only help the instructor to effectively present atopic but also help him to assess the trainee's understanding. The instructor guide enables the instructor toplan his schedule of instruction, plan the raw material requirements, day to day lessons and demonstrations.

In order to perform the skills in a productive manner instructional videos are embedded in QR code of theexercise in this instructional material so as to integrate the skill learning with the procedural practical stepsgiven in the exercise. The instructional videos will improve the quality of standard on practical training andwill motivate the trainees to focus and perform the skill seamlessly.

IMPs also deals with the complex skills required to be developed for effective team work. Necessary carehas also been taken to include important skill areas of allied trades as prescribed in the syllabus.

The availability of a complete Instructional Media Package in an institute helps both the trainer andmanagement to impart effective training.

The IMPs are the outcome of collective efforts of the staff members of NIMI and the members of the MediaDevelopment Committees specially drawn from Public and Private sector industries, various training institutesunder the Directorate General of Training (DGT), Government and Private ITIs.

NIMI would like to take this opportunity to convey sincere thanks to the Directors of Employment & Trainingof various State Governments, Training Departments of Industries both in the Public and Private sectors,Officers of DGT and DGT field institutes, proof readers, individual media developers and coordinators, but forwhose active support NIMI would not have been able to bring out this materials.

R. P. DHINGRAChennai - 600 032 EXECUTIVE DIRECTOR


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ACKNOWLEDGEMENT

National Instructional Media Institute (NIMI) sincerely acknowledges with thanks for the co-operation and

contribution extended by the following Media Developers and their sponsoring organisations to bring out this

Instructional Material (Trade Theory) for the trade of Welder under Fabrication sector.

MEDIA DEVELOPMENT COMMITTEE MEMBERS

Shri. S. Suresh Kumar - Principal,Karthikeyen ITI,Perambalur.

Shri. S. Dasarathan - Retd. Instructor,MDC Member-NIMI.

Shri. D. Justin David Raj - Instructor,Nirmala ITI,Manalikarai.

Shri. P. Manjunatha - Junior Training Officer,Govt - ITI, Mysore - 07.

Smt. G. Sangareeswari - Junioir Training Officer,Govt - ITI - Guindy.

Shri. V. Gopalakrishnan - Assistant Manager,Co-ordinator, NIMI, Chennai.

NIMI records its appreciation for the Data Entry, CAD, DTP operators for their excellent and devoted services inthe process of development of this Instructional Material.

NIMI also acknowledges with thanks the invaluable efforts rendered by all other NIMI staff who have contributedtowards the development of this Instructional Material.

NIMI is also grateful to everyone who has directly or indirectly helped in developing this Instructional Material.


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INTRODUCTION

TRADE THEORY

The manual of trade theory consists of theorectical information for the Second Semester course of the WelderTrade. The contents are sequenced accoring to the practical exercise contained in the manual on Tradepractical. Attempt has been made to relate the theoretical aspects with the skill covered in each exercise to theextent possible. This co-relation is maintained to help the trainees to develop the perceptional capabilities forperforming the skills.

The Trade Theory has to be taught and learnt along with the corresponding exercise contained in the manualon trade practical. The indicating about the corresponding practical exercise are given in every sheet of thismanual.

It will be preferable to teach/learn the trade theory connected to each exercise atleast one class beforeperforming the related skills in the shop floor. The trade theory is to be treated as an integrated part of eachexercise.

The material is not the purpose of self learning and should be considered as supplementary to class roominstruction.

TRADE PRACTICAL

The trade practical manual is intented to be used in workshop . It consists of a series of practical exercies tobe completed by the trainees during the Second Semester course of the Welder trade supplemented andsupported by instructions/informations to assist in performing the exercises. These exercises are designedto ensure that all the skills in the prescribed syllabus are covered.

The manual is divided into five modules. The distribution of time for the practical in the five modules are givenbelow.

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Module 3 Gas tungsten arc welding 140 Hrs

Module 4 Plasma arc cutting & Resistance welding 35 Hrs

Module 5 Repair and maintenance 75 Hrs

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Total 575 Hrs

The skill training in the computer lab is planned through a series of practical exercises centred around somepractical project. However, there are few instance where the individual exercise does not form a part of project.

While developing the practical manual a sincere effort was made to prepare each exercise which will be easyto understand and carry out even by below average traninee. However the development team accept that thereif a scope for further improvement. NIMI, looks forward to the suggestions from the experienced training facultyfor improving the manual.


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CONTENTS

Lesson No. Title of the Lesson Page No.

Module 1: Inspection and testing2.1.62 Inspection of weld (NDT) - Visual inspection 1

2.1.63 Measuring gauges 3

Methods of non-destructive tests 4

2.1.64 & 65 Destructive tests 8

2.1.66 Economy in welding & simple estimation 10

Module 2: Gas metal arc welding2.2.67 Safety in GMAW 12

GTAW safety 13

2.2.68 Introduction to GMAW 14

2.2.69 & 70 Introduction to GMAW equipment and accessories 16

GMAW (MIG/MAG) torches 17

2.2.71 Advantages, disadvantages of GMAW over SMAW process and applications 20

2.2.72 Process variables of GMAW 21

2.2.73 Modes of metal transfer in GMAW 23

2.2.74 Wire feed system 25

2.2.75 Welding wires used for GMAW, standard diameter and codificationas per AWS 27

2.2.76 Types of shielding gases for GMAW 29

2.2.77 Flux Cored Arc Welding (FCAW) 32

2.2.78 Edge preparation of various thickness of metals (GMAW) 34

2.2.79 GMAW defects, causes and remedies 36

2.2.80 Heat in put and techniques 40

2.2.81 Heat distribution and effects of faster cooling 43

2.2.82 & 83 Preheating and post heating treatment 44

Preheat treatment and post weld heat treatment 47

Hardening and tempering 47

Heat treatment of steels (annealing and normalising) 50

2.2.84 Submerged arc welding process principles 52

2.2.85 Electro slag welding and electro gas welding 55

2.2.86 Thermit welding 57

2.2.87 Backing strips and backing bars 59

Module 3: Gas tungsten arc welding2.2.88 Principles of Gas Tungsten Arc Welding (GTAW), advantages & limitations 61

2.2.89 GTAW process brief description 62

Advantages and disadvantages of AC and DC welding 62


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Lesson No. Title of the Lesson Page No.

GTAW process and equipment 63

Various other name of the process (Tig orgamic) 64

GTAW power sources 64

2.2.90 GTAW electrodes 68

2.3.91 GTAW torches - types, parts and their functions 71

GTAW filler rods and selection methods (criteria) 73

2.3.92 Edge preparation and GTAW parameters for different type of metals 76

2.3.93 Argon/Helium gas properties and uses 81

Defects causes and remedy 83

2.3.94 Friction welding process equipment and application 85

Laser Beam Welding (LBM) 86

Electron beam welding 87

2.3.95 Plasma arc welding 89

Module 4: Plasma arc cutting & Resistance welding2.3.96 Types of plasma arc, advantages and applications 92

2.4.97 & 98 Resistance welding process & applications and limitations 95

Module 5: Repair and maintenance2.5.99 Metallizing, types, principles equipments, their advantages and its application 98

2.5.100 Principles of operations and applications 99

2.5.101 Welding codes and standards - WPS & PQR 100

2.5.102 Reading of assembly drawing 104

2.5.103 Hard facing 106

2.5.104 &105 Surfacing/Metal build up 109

In plant training/project work 111


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LEARNING / ASSESSABLE OUTCOME

On completion of this book you shall be able to

• Test welded joints by different methods of testing. [Different methods of testing-Dye penetration test, Magnetic particle test, Nick break test, Free bend test, Filletfracture test].

• Set GMAW machine and perform welding in different types of joints on MS sheet/plate by GMAW in various positions by dip mode of metal transfer. [Differenttypes of joints Fillet (T-joint, lap, Corner), Butt (Square & V); various positions -IF, 2F, 3F, 4F, 1G, 2G, 3G].

• Set the GTAW machine and perform welding by GTAW in different types of jointson different metals in different position and check correctness of the weld[Different types of joints- Fillet (T-joint, lap, Corner), Butt (Square & V); differentmetals- Aluminium, Stainless Steel; different position- 1F & 1G].

• Perform Aluminium & MS pipe joint by GTAW in flat position.

• Set the Plasma Arc cutting machine and cut ferrous & non-ferrous metals.

• Set the resistance spot welding machine and join MS & SS sheet.

• Perform joining of different similar and dissimilar metals by brazing operation asper standard procedure [Different similar and dissimilar metals- Copper, MS,SS].

• Repair Cast Iron machine parts by selecting appropriate welding process[Appropriate welding process- OAW, SMAW].

• Hard facing of alloy steel components/ MS rod by using hard facing electrode.


1

Fabrication Related Theory for Exercise 2.1.62Welder - Inspection and testing

Inspection of weld (NDT) - Visual inspectionObjectives: At the end of this lesson you shall be able to• explain the necessity of inspection and testing of weld• describe the different stages of visual inspection• explain the check points of visual inspection• state types of testing of welds.

Necessity of inspection: The purpose of inspection is tolocate and determine the type of weld fault, strength andquality of joint and quality of workmanship.

Types of tests– Non-destructive test (NDT)

– Destructive test

– Semi destructive test

Determining the quality of the weld without destroying theweld is called a non-destructive test (NDT). The job can beused after the test. The test to be carried out on weldedspecimens by cutting the job and destroying it is calleddestructive test. The job cannot be used after the test.

Sometimes the quality of a welded joint is tested bygrinding, drilling, etching, filing etc. for finding machinability,microstructure etc. These tests are called semi-destruc-tive tests. The tested job can be used after the test byrewelding the small area damaged during the test.

Visual inspection (non-destructive test): Visual inspec-tion is observing the weld externally using simple handtools and gauges to know whether there is any externalweld defects. This is one of the important inspectionmethods without much expense. This method of inspec-tion needs a magnifying glass, a steel rule, try square andweld gauges. Visual inspection is made in three stagesnamely:

– before welding

– during welding

– after welding

Visual inspection before welding(The operator must be familiar with the type of work,electrode and welding machine)

The following factors are to be ensured.

The material to be welded is of weldable quality.

The edges have been properly prepared for welding as perthickness of the plate.

Proper cleaning of the base metal.

Setting of proper root gap.

Proper procedure to be followed to control distortion.

Proper selection of blow pipe nozzle and filler rod, flux andflame.

Proper facilities should exist for storing and drying of theelectrodes. (Fig 2)

Visual inspection during weldingThe following points are to be checked.

Studying the sequence of weld deposit.

Examining whether each weld is cleaned adequatelybefore making the next run in multi-run welding.

The following factors are to be ensured.

Polarity of the electrodes in the case of DC welding current.

Whether the cable connections are tight.

Current setting according to the size of the electrode andposition of welding.

Whether any jigs and fixtures are necessary to ensureproper alignment. (Fig 1)


2

There are two types of specimens prepared for this - one forface bend and the other for root bend. (Fig.4) This testmeasures the ductility of the weld metal in a butt joint in aplate. This test shows most weld faults quite accuratelyand it is very fast. A sample specimen can be tested ondestruction to determine (a) the physical condition of theweld and thus check on the weld procedure and (b) thewelder’s capability.

The resistance to fatigue of a welded joint is tested by fixingthe welded specimen in a chuck and rotated at a particularspeed with a load hung at the other end as shown in Fig.7.Fatigue tests are extremely useful while testing weldedshafts, cranks and other rotating parts which are subjectedto varying alternating loads.

Guided bend test: A guided bend test is one in which thespecimen as in Fig.3.

Impact test: Impact means application of a sudden forceon an object. In an impact test of a weld, a test specimen(Fig.4) is prepared from a test plate. This is furthermachined to have a V notch as in Fig.5. The test specimenwith 10 mm square cross-section is used for charpy Vimpact test and one with 11 mm diameter circular cross-section is used for the izod impact test. Fig.6 shows animpact testing machine.

The impact test is used to determine the impact value ofwelds and base metals in welded products to be used atlow temperatures up to - 400 C which are subjected tosevere dynamic loading.

Fatigue test: When a welded joint is subjected to push andpull forces alternatively for a long period, it may fail due tothe fatigue of the molecules. In this case the forces appliedwill rise to a maximum tension, decrease to zero, rise to amaximum compression and decrease again to zero. Thiscycle will be repeated which creates fatigue in the jointwhich will fail at much less loads than its maximum tensionand compression strength.

Fabrication: Welder (NSQF - 4) - Related Theory for Exercise : 2.1.62


3

Inspection after welding: Surface defects in and aroundthe welds, such as cracks, (longitudinal and transverse)(Fig.1), undercut (Fig.2), overlap (Fig.3), excessive con-vexity of contour, the weld surface smoothness of the runand penetration, control of distortion, unfilled crater are tobe inspected.

Quality of the weld metal.

Measuring gauges used for inspectionA template may be used to check the contour. (Fig.5)


Measuring gaugesObjectives: At the end of this lesson you shall be able to• describe the surface defects on weld• explain the measuring gauges• states the types of gauges.

Freedom from surface cavities and slag inclusions. (Fig 4).Deposition of runs, single or multiple.

Penetration bead in butt weld.

Use gauges for measuring both convex and concave in filletweld and to check contour of weld. (Figs 6 to 12)


4

Methods of non-destructive testsObjectives: At the end of this lesson you shall be able to• explain the non-destructive testing methods• explain the uses of the common non-destructive methods• explain the uses of special non-destructive testing.


Non-destructive testing methods are classified as com-mon testing and special testing methods.

Common non-destructive testing– Visual inspection– Leak or pressure test– Stethoscopic test (Sound)Special non-destructive tests– Magnetic particle test– Liquid penetrant test– Radiography (X-ray) test– Gamma ray test– Ultrasonic testVisual inspection: Visual inspection is the simplest,fastest, economical and most commonly used test fordetecting defects on the surface of the welded job. Theweld surface and joint are examined visually with nakedeyes preferably with the help of a magnifying lens. Visualexamination can help in detecting the following defects onthe surface of the weld.

– Porosity– Surface defects like surface cracks, external slag

inclusions, overlap, spatters, unfilled crater, misalign-ment, distortion etc.

– Undercut– Improper profile and dimensional accuracy– Poor weld appearance– Incomplete penetration.

Leak or pressure test: This test is used to test weldedpressure vessels, tanks and pipelines to determine if leaksare present. The welded vessel, after closing all its outlets,is subjected to internal pressure using water, air or kero-sene. The internal pressure depends upon the workingpressure which the welded joint has to withstand. Theinternal pressure may be raised to two times the workingpressure of the vessel. The weld may be tested as follows.

1 The pressure on the gauge may be noted immediatelyafter applying the internal pressure and again after, say,12 to 24 hours. Any drop in pressure reading indicatesa leak.


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2 After generating air pressure in the vessel, soap solu-tion may be applied on the weld seam and carefullyinspected for bubbles which would indicate leak.

Stethoscopic (sound) test: The principle of this test is thatdefect-free weld metal gives a good ringing sound whenstruck with a hammer whereas a weld metal containingdefects gives a flat sound.

An ordinary physician’s stethoscope and a hammer maybe used to magnify and identify the sound.

Structural welds and welds in pressure vessels have beensuccessfully tested using this method.

Radiographic test: This test is also called X-ray orgamma ray test.

X-ray test: In this test internal photographs of the welds aretaken. The test specimen is placed in between the X-rayunit and film. (Fig 1) Then the X-ray is passed. If there isany hidden defect, that will be seen in the film afterdeveloping it. Defects appear in the same manner as bonefractures of human beings appear in X-ray films. Below theX-ray film a lead sheet is kept to arrest the flow of X-rayfurther from the X-ray testing machine.

coloured dye is sprayed on the clean welded joint andallowed to soak. Then the dye is washed off using acleaner, and the surface dried with soft cloth.

A liquid developer (white in colour) is then sprayed on theweld. The coloured dye comes out in the shape of surfacedefects into the white developer coating. The defect can beseen in normal light with naked eyes. (Fig 4)

Gamma ray test: The short invisible rays given off byradium and radium compounds like cobalt 60 etc. areknown as gamma rays. These rays penertrate greaterthickness of steel than x-rays and the chief advantage ofthis process is portability. This test can be done at placeswhere electricity is not available. These tests are used onhigh quality jobs like boilers and high pressure vessels andpenstock pipes and nuclear vessels.

Magnetic particle test: This test is used to detect surfacedefects as well as sub-surface (up to 6 mm depth) defectsin ferrous materials.

A liquid containing iron powder is first sprayed over the jointto be tested. When this test piece is magnetised, the ironparticles will gather at the edges of the defect (crack or flaw)and can be seen as dark hair line marks with naked eyes.(Figs 2 & 3)

Liquid penetrant test; This test is based on the principlethat coloured liquid dyes and fluorescent liquid penetrateinto the cracks and are used to check for surface defectsin metals, plastics, ceramics and glass. A solution of the

Ultrasonic test: Sound waves of high frequency are usedin this test. This test is used to find out the discontinuitiesin the weldment. The sound waves can penetrate from avery small thickness of plate to 6 to 10 metres of steel.

A sound wave producing transmitter is placed on the job.The echo of the sound waves is directly shown on thecalibrated screen attached with the ultrasonic testing unit.(Fig 5)



6 Fabrication: Welder (NSQF - 4) - Related Theory for Exercise : 2.1.63

Guided bend test: A guided bend test is one in which thespecimen as in Fig. 6.

There are two types of specimens prepared for this - one forface bend and the other for root bend. (Fig. 7) This testmeasures the ductility of the weld metal in a butt joint in aplate. This test shows most weld faults quite accuratelyand it is very fast. A sample specimen can be tested ondestruction to determine (a) the physical condition of theweld and thus check on the weld procedure and (b) thewelder’s capability.

Impact test: Impact means application of a sudden forceon an object. In an impact test of a weld, a test specimen(Fig. 8) is prepared from a test plate. This is furthermachined to have a V notch as in Fig.8. The test specimenwith 10 mm square cross-section is used for charpy V

impact test and one with 11 mm diameter circular cross-section is used for the izod impact test. Fig.9 shows animpact testing machine.

The impact test is used to determine the impact value ofwelds and base metals in welded products to be used atlow temperatures up to - 400 C which are subjected tosevere dynamic loading.

Fatigue test: When a welded joint is subjected to push andpull forces alternatively for a long period, it may fail due tothe fatigue of the molecules. In this case the forces appliedwill rise to a maximum tension, decrease to zero, rise to amaximum compression and decrease again to zero. Thiscycle will be repeated which creates fatigue in the jointwhich will fail at much less loads than its maximum tensionand compression strength.


7Fabrication: Welder (NSQF - 4) - Related Theory for Exercise : 2.1.63

The resistance to fatigue of a welded joint is tested by fixingthe welded specimen in a chuck and rotated at a particularspeed with a load hung at the other end as shown in Fig.10Fatigue tests are extremely useful while testing weldedshafts, cranks and other rotating parts which are subjectedto varying alternating loads.


8

Fabrication Related Theory for Exercise 2.1.64 & 2.1.65Welder - Inspection and testing

Destructive testsObjectives: At the end of this lesson you shall be able to• explain the necessity for destructive tests• explain the different methods of destructive test of weldments• explain the advantages and limitations of workshop and laboratory tests• identify the specimen for destructive tests.

Introduction: Welded joints are tested without damagingor destroying the welded structure under non-destructivetesting methods which were explained earlier. Now to knowthe property of material used for welding and to know thestrength of a weld joint and also to judge the skill of thewelder, a destructive test is to be performed on a weldedspecimen which was destroyed during the testing. Thereare two main methods of destructive testing. They are:– workshop tests

– laboratory tests

Workshop tests

These are the tests that can be performed in the workshop.

– Nick break test

– Free bend test in a vice

– Fillet fracture test (by using a bending bar)

Nick break test: In a nick break test a saw cut of about1.5mm to 2mm depth is made along the centre line of theweld, and a hammer blow is given on the reverse of the jointas shown in the figure. (Fig 1). The joint will break along thesaw cut and by observing the fractured surface, variousdefects like slag inclusions, lack of fusion, lack of penetra-tion, etc. can be identified.

Free bend test: The welded joints are fixed on a vice andbent by applying forces by hammer/bending bar to deter-mine the defect in the weld done by a trainee in a workshop.(Figs 2&3) The workshop tests are usually used to breakopen the weld in a workshop using a vice and hammer forvisual inspection.

Advantages and limitations: The time taken to performthe test is less. Cost of testing is less. This test is usefulfor testing the welders in the beginning when the weldcontains many defects. Does not give the actual strengthof the joint. Cannot be used for testing the quality of weldconsumables. (electrodes and filler rods)

Examination of fractured weld: The fractured weld mayexhibit and show the following internal defects. (Figs 4, 5,6,and 7)

– Lack of fusion– Incomplete penetration– Slag inclusions– Blow-holes or porous weldLaboratory testsThe laboratory tests conducted on welds are the:


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– tensile test– guided bend test– impact test– fatigue test.

Tensile test: A tensile test is conducted to know thetensile strength and ductility (i.e. elongation) of a weld.

Two types of test specimens are prepared for the tensiletest.

They are– transverse tensile test specimen (Fig 8)

– all-weld metal tensile specimen. (Figs 9 and 10)

Fabrication: Welder (NSQF - 4) - Related Theory for Exercise : 2.1.64 & 2.1.65

The tensile test gives the values of the tensile strength ofthe weld and the percentage of elongation of the weld. Thisreveals the suitability of a joint welded with certain elec-trodes and base metals for a particular service condition.


10


Economy in welding & simple estimationObjectives: At the end of this lesson you shall be able to• describe the method of cost estimation• explain about economy in welding• explain the principle, application, advantages and limitations of electron beam, electro slag, friction and

laser beam welding• describe the parts of the above advanced welding equipment.

The following factors are to be considered for cost estima-tion.

Material cost: Material cost involves the cost of all basicmaterials such as steel sheets, plates, rolled sections,forgings, angle irons, forgings, casting etc. as may beused.

Fabrication cost: Fabrication cost involves cost of (1)preparation (2) welding and (3) finishing.

Preparation cost: Preparation cost involves cost of mate-rial handling, cutting, machining or shearing plates orsections, preparing the edges for welding, forming, fittingup, positioning, labour for these operations etc.

Welders should ensure that the plates and sections areprepared for welding, either by machining or by flamecutting in accordance with the recommendations of thedesign office.

The effects of inaccurate edge preparation and poor fit upresulting in extra welding and the consequent additionalwelding costs are illustrated in Figs 1 and 2.

Welding cost: The welding cost involves the cost ofelectrodes, power consumed, welding labour etc.

In determining the direct welding cost, the following factorsare taken into consideration.

– Cost of electrodes - this being dependent on the typeand size of electrode and edge preparation employed.

– Power consumed

where V = Voltage, A = Current in amperes

T = Welding time in minutes

E = Efficiency of the machine.

E is assumed to be 0.6 in the case of a welding transformerand 0.25 in the case of a welding generator.

– Speed of welding– Welding labour cost (Fig 3)– Position of welding

Finishing cost: Finishing cost involves cost of all postwelding work, such as machining, grinding, sand-blasting,pickling, heat treatment, painting etc. and the labourinvolved in carrying out these operations.

Overhead cost: Overhead costs involve all other costs,such as office and supervisory expenses, lighting, depre-ciation on capital, etc. which are not to be directly chargedto a job. There exists an elaborate and accurate systemof computation and allocation of overhead expenses to thevarious stages of manufacturing process.


11

Economics of welding: Over-welding, that is excessivebuild up in the case of butt weld and fillet welds larger thanthose specified, should always be avoided. (See sizecomparison in Fig 4)

Ensure that the largest size of electrode compatible withthe plate thickness is used. Use of smaller electrode willincrease labour hours and excessive distortion.

Use proper welding current. Excessive current will lead toexcessive spatter loss and unsatisfactory weld.

Avoid excessive stub end loss; ensure that most of theusable portion of the electrode is used. The stub end shouldnever be more than 50mm. (Fig 5)

The most convenient position of welding is in the down hand(flat) position. Whenever possible welding should be carriedout in the flat position. A graphic form of the relative costand speed of welding is shown in Fig 6 & 7.



12

Fabrication Related Theory Exercise 2.2.67Welder - Gas metal arc welding

Safety in GMAWObjective: At the end of this lesson you shall be able to• safety precaution to be followed in the process.

Safety in GMA welding/CO2 weldingThe general safety precautions for arc welding (SMAW)are also applicable to GMAW.

Ultra violet lightDuring MIG welding Ultra Violet Light production is at thehigher end of the scale and suitable eye protection must beused.

Adequate eye protection should always be worn. If weldingfor long periods, flash goggles with A#12 lens shade shouldbe worn under the arc helmet. A#11 lens is recommendedfor nonferrous GMAW and A#12 for ferrous GMAW. Allwelding should be done in booths or in areas protected bycurtains. This is done to protect others in the weld area fromarc flashes.

HeatWelding in any form produces heat which can cause burnsand the possibility of fire.

Suitable clothing must be worn. This is done to protect allparts of the body from radiation or hot metal burns. Leatherclothing offers the best protection from burns.

FumesFumes from the MIG welding process are produced by theburning of contaminants on the surface of the materialbeing heated.

The MIG welding of galvanised metal is extremely dangerousto the operator because of zinc poisoning unless suitableprotection is used.

Ventilation should be provided. This ventilation and/orfiltering equipment is necessary to keep the atmospherearound the welder clean. Carbon monoxide is generatedwhen doing GMAW and using CO2 as a shielding gas. It issuggested that all welding be done in well ventilated areas.

Ozone is also produced when doing GMAW and ozone isa highly toxic gas. Metals still covered with chlorinatedhydrocarbon solvents will form poisonous, toxic phosgenegas when welded.

Protect arc cables from damage. Do not touch uninsulatedelectrode holders with bare skin or wet gloves. A fatalshock could result. Welding in wet or damp areas is notrecommended.

Shielding gas cylinders must be handled with caution.

Welding environment safety rules– keep the welding area clean

– keep combustibles out of the weld area

– maintain good ventilation in the weld area

– repair or replace damaged power cables

– make sure the part to be welded is securely grounded/earthed

– welding helmets should have no light leaks. Should nothave scratches or cracks

– use the proper colored lens with correct shade numberin the helmet

– wear safety glasses when grinding

– do not see the arc with bare eyes

– use safety screens or shields to protect your area

– wear proper clothing. Your entire body should becovered to protect you from arc radiation


13

GTAW safetyObjective: At the end of this lesson you shall be able to• state the safety precautions to be followed in TIG welding process.

GTAW/TIG welding is a skill which may be performedsafely with a minimum of risk if the welder used goodcommon sense and safety rules. It is recommended thatyou establish good safety habits as you work in thisindustrial area. Check your equipment regularly and besure that your environment is safe. Safety in TIG weldingcovers the following major areas and includes

Electrical current: Primary current to the electrical pow-ered welding machine is usually 220V A/C or more, andthis amount of voltage can cause extreme shock to thebody and possible death. For this reason

– never install fuses higher than specified

– always ground/earth the welding machine properly

– install electrical components as per the codes given bythe electricity boards

– ensure electrical connections are tight

– never open a welding machine when it is operating

– lock primary voltage switches, open and remove fuseswhen working on electrical components inside themachine

– welding current supplied by the power supply has amaximum of 80 open circuit volts. At this low voltage,the possibility of lethal shock is very small. However,it will still produce a good shock. To reduce thepossibility of this occurrence.

– keep the welding power supply dry

– keep the power cable, ground cable and torch dry

– do not weld in damp area. If you must, wear rubberboots and gloves

– make sure the ground clamp is securely attached tothe power supply and the work piece

– High frequency components in some GTAW ma-chines produce a spark for starting the initial arc ormaintenance of the arc during the alternating currentwelding. The high frequency voltage is very high,however the amperage is very low. Since the amper-age is so low, the high frequency voltage will notusually travel through the body and is therefore notas dangerous as other currents.

Inert gases: Inert gases used in GTAW are produced anddistributed to the user in two forms: high pressure gas andliquid. All storage vessels used for inert gases are approvedby the department of transportation and are so stamped onthe vessel name plate or the cylinder wall.

Most of the gases used in GTAW are inert, colourless andtasteless. Therefore special precautions must be takenwhen using them. Nitrogen, argon,and helium are nontoxic. However they can cause asphyxiation (suffocation)in a confined or closed area that does not have adequateventilation. Any atmoshphere that does not contain atleast 18 % oxygen can cause dizziness, unconscious-ness or even death. The gases cannot be detected by thehuman senses and will be inhaled like air. So ensure thewelding area is well ventilated with good air circulation.

Welding environment safety rules– keep the welding area clean

– keep combustibles out of the weld area

– maintain good ventilation in the weld area

– repair or replace damaged power cables

– make sure the part to be welded is securely grounded/earthed

– welding helmets should have no light leaks. Should nothave scratches or cracks

– use the proper colored lens with correct shade numberin the helmet

– wear safety glasses when grinding

– do not see the arc with bare eyes

– use safety screens or shields to protect your area

– wear proper clothing. Your entire body should becovered to protect you from arc radiation

– when welding on cadmium coated steels, copper orberyllium copper use special ventilation to removefumes from the weld area.



14

Fabrication Related Theory for Exercise 2.2.68Welder - Gas metal arc welding

Introduction to GMAWObjectives: At the end of this lesson you shall be able to• state the main difference between shielded metal arc welding and Co2 welding• state the principle of CO2 welding.

Introduction to CO2 welding: Fusion welding of metalplates and sheets is the best method of joining metalsbecause in this process the welded joint will possess thesame properties and strength as the base metal.

Without a perfectly shielded arc and molten puddle, theatmospheric oxygen and nitrogen will get absorbed by themolten metal. This will result in weak and porous welds.

In shielded metal arc welding (SMAW) the arc and moltenmetal are protected/shielded by the gases produced by theburning of the flux coated on the electrode.

The above mentioned shielding action can be done bypassing an inert gas such as argon, helium, carbon-di-oxide through the welding torch/gun. The arc is producedbetween the base metal and a bare wire consumableelectrode fed continously through the torch.

Principle of GMA welding: In this welding process, anarc is struck between a continuously fed consumable barewire electrode and the base metal. The heated base metal,the molten filler metal and the arc are shielded by the flowof inert/noninert gas passing through the welding torch/gun. (Fig.1)

If an inert gas is used to protect the arc produced by aconsumable metal electrode, this process is called MetalInert Gas Welding (MIG).

When carbon-dioxide is used for shielding purposes, it isnot fully inert and it partly becomes an active gas. So CO2welding is also called as Metal Active Gas (MAG) welding.

MIG/MAG welding is a name with respect to gasused for shields purposeOn the other hand Gas Metal Arc Welding is thecommon name.

Basic equipment for a typical GMAW semiautomaticsetup: (Fig 2)• Welding Power Source - provides welding power.

• Wire Feeders - controls supply of wire to welding gun.

• Supply of Electrode Wire.

• Welding Gun - delivers electrode wire and shieldinggas to the weld puddle.

• Shielding Gas Cylinder - provides a supply of shieldinggas to the arc.

Other names• MIG (Metal Insert Gas) welding,

• MAG (Metal Active Gas)/CO2 Welding

• GMAW (Gas Metal Arc Welding)

GMAW can be done in three different ways:

• Semiautomatic welding - equipment controls onlythe electrode wire feeding. Movement of welding gun iscontrolled by hand. This may be called hand-held weld-ing.

• Machine welding - uses a gun that is connected to amanipulator of some kind (not hand-held). An operatorhas to constantly set and adjust controls that movethe manipulator.

• Automatic welding - uses equipment which weldswithout the constant adjusting of controls by a welderor operator.

On some equipment, automatic sensing devices con-trol the correct gun alignment in a weld joint.




16

MIG welding power sources have come a long way fromthe basic transformer type power source to the highly elec-tronic and sophisticated types we see around today.

Even though the technology of MIG welding has changed,the principles of the MIG power source have, in most cases,not. The MIG power sources use mains power and con-verts that mains power into CV (constant voltage), DC(direct current) power suitable for the MIG welding pro-cess.

MIG welding power sources control voltage – this is doneby either voltage stepped switches, wind handles, or elec-tronically. The amperage that the power source producesis controlled by the cross sectional area of the wire elec-trode and the wire speed, ie the higher the wire speed foreach wire size, the higher the amperage the power sourcewill produce.

Because the output of the MIG power source is DC (directcurrent) the terminals on the front will have + positive andnegative on the output side. The principles of electric cir-cuits states that 70% of the heat is always on the positiveside.

This means that the lead that is connected tothe positive side of the welder, will carry 70%of the total energy (heat) output.

The characteristics volt, ampere curves (A & B) are shownin Fig.1.

Curve A ( For SMAW): On the output slope or voltamperecurve A, a change from 20 volts to 25 volts will result in adecrease in amperage from 135 amps to 126 amps. With

Fabrication Related Theory for Exercise 2.2.69 & 70Welder - Gas metal arc welding

Introduction to GMAW equipment and accessoriesObjective: At the end of this lesson you shall be able to• state the power sources for GMAW

a change of 25 percent in voltage, only a 6.7 percentchange occurs in the welding current in curve A. Thus if thewelder varies the length of the arc, causing a change involtage, there will be very little change in the current and theweld quality will be maintained. The current in this machine,even though it varies slightly is considered constant.

This is called drooping characteristic power source. Alsocalled constant current (CC)power source.

This type of power source is used in SMAW & GTAWprocess.

Curve B (For GMAW): The open circuit voltage curve fora setting of 50 volts on the machine is shown as curve B inthe Fig.1. The same 20 volt to 25 volt (25 percent) changein the welding voltage will result in a drop in current from 142amps to 124 amps or 13.3 percent. This slower sloping voltampere curve output causes a large change in amperagewith the same small change in voltage. A welder may wishto have this slower sloping (flatter) volt-ampere outputcurve.

This is called flat characteristic power source. Also calledconstant Voltage(CV)power source.

This type of power source is used in GMAW & SAWprocess.

With a flatter output slope the welder can control the moltenpool and electrode melt rate by making small changes inthe arc length. Control of the molten pool and electrodemelt rate are most important when welding in the horizon-tal, vertical and overhead positions.


17

MIG/MAG torch connectionThe torch connection is the system in which the MIG torchis connected to the wire feeder. There are various types ofMIG torch connections. Different manufacturers can useany one of many systems to connect their torch to thewire feeder.

When ordering a new Torch tell the supplier

a the type of torch you need, including amperage rating

b the type of connection on the feeder so the torch canbe supplied to match the connection

The Torch connection is also the area where the wire elec-trode, welding current and welding gases are passed ontothe welding torch. This means these components shouldbe checked for damage or leaky seals etc, so the con-nection will do its job correctly.

MIG/MAG torchesThe MIG Torch is connected to the wire feeder, and its job

is to deliver the wire electrode, shielding gas and the elec-trical welding current to the welding area. There are a lotof different shapes and styles of MIG Torch out in themarketplace but they all have things in common.(Fig. 1 & 3).

GMAW (MIG/MAG) torchesObjective: At the end of this lesson you shall be able to• state the types and functions of torches.

Fabrication: Welder (NSQF - 4) - Related Theory for Exercise : 2.2.69 & 70

1 Aircooled (less than 200 Amps) or watercooled (above200 Amps) (Fig 2)

2 Current rating. The operator must select the correctsize Torch. Using a torch that is not sufficiently ratedfor the machine may result in the Torch overheating.This may result in a poor weld and damage to the torch.A Torch with an excessive rating will be larger andheavier than the smaller Torch, which could result indiscomfort for the operator.

3 They all have parts that will wear out (consumables egliners, tips, diffuser, nozzle, etc.)

Let’s take a look at each part (Fig 4)

Liner: The liner causes the most problems. First, theyhave a life span that is approximately one to four rolls ofMIG wire depending on the quality of the liner and wire.


18 Fabrication: Welder (NSQF - 4) - Related Theory for Exercise : 2.2.69 & 70

The liner length is most important. In the field it is verycommon to find even newly fitted liners that have been cuttoo short. This results in the wire being able to move aroundbehind the welding tip and leading to bad wire feeding.The liner has to be fitted correctly and different MIG Torchwill often have a different way of ending up with a liner thatis the correct length.Don’t just take out the old liner and cut the new one to thesame length. It could end up with an incorrect result. Pleaserefer to MIG Torch manual.All MIG Torch should be laid out straight ont he floor be-fore trimming the liner, to prevent the new liner being cuttoo short. Do not cut the liner if the torch lead is coiled up.Gas diffusers The gas diffuser’s job is to make sure thatthe shielding gas is delivered to the shielding nozzle cor-rectly. It is designed to make the gas come out as straightas possible and equally supplied around inside the gasshield nozzle. Diffusers can be made of different materi-als, eg copper, brass or fibre. Some diffusers will also bethe tip holder.Contact tip holder This is the item which holds the weld-ing tip in place. Again, tip holders can be very different in

The life of the liner will also be increased if the operatorremoves and cleans it by soaking in non-corrosive and anon-toxic solvent. Each wire size needs to have the cor-rect wire size liner. Be aware some liners may fit morethan one size of wire.

There are also different materials for different types of wireelectrode, eg steel or stainless liners for solid wiresand Teflon liner for aluminium.

design and are very often unique to that brand of MIGtorch.

Contact tips The Contact tip/tube is the key to good weld-ing. First of all, it is the way that welding amperage isdelivered to the welding wire electrode, often with a veryhigh amperage.

Most contact tips are made of copper alloy, the better thealloy the better the tip will pass current to the wire elec-trode and the less wear the MIG tip will have; also the lessthe tip will oxidize.

The size is important. The right size contact tip must beselected contact. If the selected tip size is too large thewire electrode will not make a good contact, leading topoor welding performance.

If a contact tip selected is too small, the wire electrodewill feed poorly and may even jam in the contact tip.

Nozzle: Guns are available with a straight or curvednozzle. The curved nozzle provides easy access to intricatejoints and difficult-to-weld.

Torch angleThe position of gun and electrode with respect to the jointaffects the weld bead shape and penetration rather than arcvoltage or travel speed. The gun is usually maintainedwithin 10 - 20º on either side of the vertical. Depending onwhich way the gun is incline, the technique is referred to asforehead and backhand. The various electrode positionsand techniques and their effects are shown in Fig 5. It isobserved that as the electrode is changed from perpendicularto the forehand technique, the weld bead becomes shallowerand wider and has less penetration.

Backhand technique gives a more stable arc, less spatterand a narrower, more convex weld bead with deeppenetration. Perpendicular technique is used more inautomatic welding and avoided in semi-automatic modebecause the end of the gas nozzle restricts the operator’sview of the weld pool.


19Fabrication: Welder (NSQF - 4) - Related Theory for Exercise : 2.2.69 & 70

Synergic ControlThe complexity of setting welding parameters in conven-tional DC and pulsed GMAW promoted the development ofequipment with ‘Single-knob’ controls known as SynergicControl. These systems relied on selection of combina-tions of present welding (e.g. Wire feed speed/meancurrent and voltage) by means of a single control.

This is possible now because of development of electronicpower regulation and micro processor control andprogrammable equipment which can supply a large numberof predetermined welding conditions as well as allowingusers to record and retrieve their own customerisedparameters.

Although in the pulsed GMAW process the optimumwelding parameters can be accurately predetermined, if achange in mean current is required the control settingsmust be recalculated and a number of the welding param-eters reset. This could impose significant practical prob-lems including the possibility of error and resultant deterio-ration in operating performance. Fortunately it is possibleto store both the predetermined parameters and the controlequations in the equipment and automatically adjust theoutput in response to a single input signal. This system isknown as Synergic Control (Fig. 6).


20


Advantages, disadvantages of GMAW over SMAW process and applicationsObjectives: At the end of this lesson you shall be able to• state the advantages and disadvantages of Co2 welding over shielded metal arc welding process• state the applications of Co2 welding.

Advantages: Welding is economical due to less edgepreparation and no stub loss.

Produces joints with deep penetration.

Thin and thick materials can be welded.

It can be used for welding of carbon steels, alloy steel,stainless steel, copper and its alloys, aluminium and itsalloys.

Welding in all positions can be done.

Deposition rate is more.

No solid flux is used. So needs no cleaning of slag aftereach run.

Reduced distortion.

DisadvantagesWelding equipment is costly, more complex and lessportable.

Since air drifts may disturb free flow of the shielding gas,GMAW may not work well in outdoor welding.

Applications: This process can be used for weldingcarbon, steel alloy steels, stainless steel, aluminium,copper, nickel and their alloys, titanium etc.

Light and heavy fabrication work.

This process is successfully used in ship building fabrica-tion of pressure vessels and automobile industries.


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GMA welding process parameters/variablesThe following parameters must be considered in thewelding procedure of GMAW/CO2 welding.

Electrode sizeRate of wire feed (Welding current)Arc voltageStick outwelding positionShielding gasTravel speedelectrode positionElectrode: Best results are obtained by using the propersize wire for the thickness of the metal to be welded and theposition in which the welding is to be done.

Electrode wires should be of the same composition as thatof the material being welded.

Basic wire diameters are 0.8 mm, 1.0 mm, 1.2 mm, 1.6 mmand 2.4 mm.

Welding current: The wire feed speed will control thecurrent. A wide range of current values can be used witheach wire diameter. This permits welding metal of variousthicknesses without having to change the wire diameter.The current selected should be high enough to secure thedesired penetration and low enough to avoid under-cuttingor burn through.

The success of GMA welding is due to the concentrationof high current density at the electrode tip.

General data on current selection is given in the table givenbelow.

The current varies as the wire feed varies.

Arc voltage: This is a very important variable in GMAW/CO2 welding process, mainly because it determines thetype of metal transfer by influencing the rate of droplettransfer across the arc. The arc voltage to be used dependson the base metal thickness, type of joint, electrodecomposition and size, shielding gas composition, weldingposition, type of weld and other factors.

For details refer to the table of General guide to weldingconditions.

Arc travel speed: The linear rate at which the arc movesalong the joint, termed arc travel speed, affects the weldbead size and penetration.

If the arc travel speed is lowered, the weld pool becomeslarger and shallower. As the travel speed is increased, theheat input rate of the arc is decreased; consequently thereis decreased penetration and narrower weld bead. Whenthe travel speed is excessive, undercutting occurs alongthe weld bead, because the deposition of the filler metal isnot sufficient to fill the paths melted by the arc.

Stick out: It is the distance between the end of the contacttube and the tip of the electrode. (Fig 1)


Process variables of GMAWObjectives: At the end of this lesson you shall be able to• state the type of edge preparation done for CO2 welding• state and explain different welding parameters to be set for CO2 welding• explain the welding procedure while using CO2 welding process.

Ranges of wire feed rate in CO2 welding (Current is shown in brackets)Wire feed speed, m/min]

Wire dia. (mm) Spray type arcs (28 - 32 V) Short circuiting arcs (16-22 V)

0.8 5.0-15 (150-250 amps) 2.5-7.5 (60-160 amps)1.2 5.0-15 (200-350 amps) 2.0-3.8 (100-175 amps)1.6 5.0-8.8 (350-500 amps) 1.5-2.0 (120-180 amps)

Too long a stick out results in excess weld metal beingdeposited at low arc heat, giving rise to badly shaped weldand shallow penetration.

When the stick out is too short, excessive spatter getsdeposited on the nozzle, which can restrict the shieldinggas flow and cause porosity in the weld.


22

Recommended stick out is 6 to 13 mm for a shortcircuiting arc, and 13 to 25 mm for the spraytransfer arc.

Electrode position: In all welding processes, the positionof the gun and electrode with respect to the joint affects theweld bead shape and penetration. The welding can be doneeither by using Forehand/Forward technique or by usingBackhand/ Backward technique. The gun angles areusually maintained within 10 to 15° as shown in Fig 2.



23

Types of metal transfer: In GMAW/CO2 welding process,the weld metal is transferred from the electrode wire to thebase metal in different methods/modes. Though there aremany methods, only the following four methods are usedpopularly used in industries.

– Spray transfer (Free flight)

– Globular transfer (Intermediate)

– Short circuit or Dip transfer

– Pulsed transfer

The type of metal transfer that occurs will depend on theelectrode wire size, shielding gas, arc voltage and weldingcurrent.

Spray transfer: In spray transfer very fine droplets of theelectrode wire are rapidly projected through the arc from theend of the electrode to the workpiece. (Fig.1) Spray transferrequires high current density (28 to 32V).

To obtain a good spray mode of welding shielding gasescontaining a blend of argon is used. The spray method ofmetal transfer can be used with most of the common weld-ing wire electrodes (eg mild steel, aluminium, stainlesssteel).

The advantages of metal spray transfer arei high deposition ratesii good travel speedsiii good looking weld appearanceiv little weld spatterv good weld fusionvi very good on heavy sectionsThe disadvantages of the spray mode arei higher capacity power source neededii weld position is limited to flat and horizontal filletiii the cost of using a more expensive mixed gasiv higher radiated heat is produced so extra protection is

needed

Globular transfer: In globular transfer, only a few dropsare transferred per second at low current values, whilemany drops are transferred at high current values. Thistransfer occurs when the welding current is low. (Fig. 2).The voltage range is 23 to 27V.

The spatter produced in this transfer is more and hence itis less preferred. But this is a good transfer method forusing CO2 gas as a shielding gas.

Short circuit transfer (DIP transfer): In short circuittransfer, as the molten wire is transferred to the weld, eachdrop touches the weld puddle before it breaks away fromthe advancing electrode wire. The circuit is shorted and thearc is extinguished. (Fig 3). The voltage range is 16 to 22V.

It permits welding thinner sections with greater ease, andis extremely practical for welding in all positions.

Pulsed spray transfer (Fig 4)Pulsed spray transfer has a steady stream of metal drop-lets crossing the welding arc. The pulsed power sourcesupplies the welding arc with two types of welding cur-rent.


Modes of metal transfer in GMAWObjective: At the end of this lesson you shall be able to• state and explain the different types of metal transfer in CO2 welding.


24

1 Peak current - this current allows the formation ofmetal droplets which then cross the welding arc.

2 Background current - the background current will keepthe arc alive, but doesn’t allow for any weld metal trans-fer.

Pulsed spray transfer allows time for the weld puddle tofreeze a little on the background current cycle, which al-lows for

i more control of the weld puddle.

ii more time for impurities to float to the top of the weldpool resulting in cleaner and stronger welds.

Advantagesi able to spray thinner metals

ii less heat input

iii stronger welds

iv more weld control

v out-of-position welding

vi Little spatters

Disadvantagesi higher set up costs

ii needs operator training

iii lower deposition rate



25


Wire feed systemObjective: At the end of this lesson you shall be able to• state the functions of wire feeder and different types of drive rollers.

Wire feeder (Fig 1)The wire feeder is the part of the MIG/MAG welding set upthat:

i) Controls the speed of the wire electrode and pushesthis wire from the feeder through the welding torch tothe workpiece.

ii) Provides the path for welding current to be passed fromthe welding power source through the interconnectinglead to the feeder and then to the welding torch.

iii) Provides gas flow control through a solenoid valve. Thegas is fed down from the gas regulator to the weld areavia the feeder and then the MIG welding torch.

Wire feeders come in many different shapes and sizes,but they all do the same basic job roles. Feeders can beseparated from the power source or built into the powersource itself. Feeders are made up of different parts, eachhaving a different job role.

Wire spool holder. This is designed to hold the spool ofthe correct wire size in place on the feeder to ensure thewire electrode is on the correct input angle for the driveroller to be able to do its job properly.

Drive motor MIG/MAG welding relies on smooth and con-stant wire feed. The wire drive motor has the job of turningthe drive rollers (this can be one or more sets of rollers).Undersize drive motors can result in poor feeding of thewire electrode down the MIG welding torch . This will havethe effect of making the overall performance of the MIGmachine sub-standard as compared to a machine with aquality drive system.

Drive rollers: The drive rollers grasp the wire electrodeand continuously feed the wire down the MIG torch intothe welding arc (Fig 2 & 3). The rollers need to be se-lected by :

i) the wire size

ii) the type of wire to be fed. Each type of wire may needa different style of roller groove – eg

V rollers for steel and other hard wires

V-Knurled for Fluxcored wire

U-Grooved for aluminium and other soft wires

The idea of using the correct roller is to have a good wiredrive without crushing the wire. The pressure roller is alsoused to set the wire tension. This must be set with enoughpressure to feed the wire electrode, but not too much ten-sion as to crush the wire.

iii) all guides must be as close as possible to the driveroller to prevent the possibility of the wire bunching up.

Wire feed controlsThe wire feeder will have its own built-in control system.The number of controls that will be built into the feeder willdepend on the type of feeder but the most common are


26

i) Wire speed - this control is the adjustment for howfast the drive rollers will turn and as stated earlier, thefaster the wire speed for each wire size the more amper-age the power source will produce. The wire speed con-trols can be labelled as wire speed, eg ipm (inches perminute) or mpm(metre per minute), or as a percentagefrom the slowest speed being zero to the highest speedbeing 100%. Usually mpm will be the range of 1 m/min to25 m/min.

The amperage being set by the wire speed setting willalso have an effect on the speed of travel and the deposi-tion rate of the wire (how fast the weld metal is being putonto the weldpiece); with the advantage of, the higher theamperage the thicker the material that can be welded.

ii) Purge switch - Some feeders have a purge switch.This is to allow the gas flow setting to be set on the gasregulator without turning of the wire feed roller or withoutany welding power being turned on.

iii) Burnback - Burnback is the setting of the degree thatthe wire electrode will melt back towards the contact tipat the completion of the weld. If there is too much burnbackthe wire electrode will melt back onto the contact tip, pos-sibly damaging it. If there is not enough burnback set, thewire electrode will not melt away from the weldpool andcan be left stuck to the weld metal.

iv) Spot timers or stitch modes are to be found on somefeeders. These controls normally control the time the driveroller will turn for after the trigger contactor has been acti-vated.



27

Electrode wire - consumable wire for GMAW: Perform-ance & metal transfer characteristics are largely governedby the diameter of the wire and the machine settings suchas arc voltage and amperage and chemical properties ofthe filler wire employed.Machine settings: Diameter of the wire and ampere/current employed for welding decide the type of metaltransfer. The various recommended diameter, voltage andcurrent ranges are tabulated in tables below for weldingmild steel, low alloy steel and stainless steel.Approx. machine settings for short circuit metal transfer onmild and low alloy steel

Electrode Arc Amperagediameter(mm) voltage range0.8 16-22 80-1901.2 17-22 100-225

Approx. machine settings for spray arc transfer on mild andlow alloy steel

Electrode Arc Amperagediameter(mm) voltage range0.8 24-28 150-2651.2 24-30 200-3151.6 24-32 275-500

Approx. machine settings for short circuit transfer onseries 300 stainless steel

Electrode Arc Amperagediameter(mm) voltage range0.8 17-22 50-1801.2 17-22 100-210

Approx. machine settings for spray transfer on series 300stainless steel


Welding wires used for GMAW, standard diameter and codification as perAWSObjective: At the end of this lesson you shall be able to• state the chemical composition of different electrode wires.

Electrode Arc Amperagediameter(mm) voltage range0.8 24-28 160-2101.2 24-30 200-3001.6 24-32 215-325

Chemical properties: Chemical compositions of the fillerwire play a very important role. The main composition,apart from the major elements, in the case of mild steelwelding, will contain deoxidisers like Si, Mn to take of careof porosity due to oxidation of carbon in the steel. Typicalcomposition of mild steel filler wires are listed in the table.We are using ER70S-6 for most of our carbon steelfabrication.Specification of electrode wiresThe GMAW electrode specification as per AWS is as givenbelow.Eg: E 70S-2 or ER70S-2 or E70T-2E — ElectrodeER — Electrode can also be used as a filled Rod in GTAW.70 — 70 x 1000 PSI — Tensile strength of the weld metalin pounds per square inch.S — Solid wire / RodT — Tubular wire used in FCAW.2 — Chemical composition of the wire.

Chemical composition, Weight percent

AWS classification c Mn Si P S Cu Ti Zr Al

70S-2 0.07 0.025 0.035 0.5

70S-3

70S-6

0.90to1.40

0.40to1.40

0.05to0.15

0.02to0.12

0.05to0.15

0.06to0.15

0.90to1.4

0.45to0.7

0.07to0.15

1.4to1.85

0.8to1.15



Wire electrodes selectionThe selection of the wire electrode to be used in the MIG/MAG process is a decision that will depend on

1 the process being used (eg, solid wire or fluxcore wire)

2 the composition of the metal being welded

3 welding indoors or outdoors

4 joint design

5 cost

6 mechanical properties of the weld material and thosethat are a match for the base material.


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Argon used with the gas metal arc spray transfer processtends to produce deeper penetration through the center lineof the bead. Spray transfer occurs more easily in argonthan in helium.

Reactive gases and gas mixtures used in GMAWCarbondioxide: Carbondioxide (CO2) has a higher ther-mal heat conductivity than argon. This gas requires ahigher voltage than argon. Since it is heavy, it covers theweld well. Therefore less gas is needed.

CO2 gas is cheaper than argon. This price difference willvary in various locations. Beads made with CO2 have a verygood contour. The beads are wide and have deep penetra-tion and no undercutting.

There are three types of shielding gases used for GMAW.They are inert gases, reactive gases and gas mixtures.

Inert gases: Pure argon and helium gas are excellentfor protecting the arc, metal electrode and weld metal fromcontamination. Argon and helium are generally used forGMAW of non ferrous metals. Helium has very goodconductivity and conducts heat better than argon. There-fore helium is choosen for welding thicker metals as well ashigh conductivity metals like copper and aluminium.

For thinner metal welding, lower conductivity argon is thebetter choice. Also argon is often used for welding out ofposition because of its lower thermal conductivity. Argongas is 10 times heavier than helium gas, hence less argongas is required to provide a good shield as compared tohelium gas.

The weld bead contour and penetration are also affected bythe gas used. Welds made with argon generally havedeeper penetration. They also have a tendency to under cutat the edges. Welds made with helium have wider andthicker beads. Fig.1 shows the shape of welds made withvarious gases and gas mixtures.

The arc in a CO2 atmosphere is unstable and a great dealof spattering occurs. This is reduced by holding a short arc.Deoxidizers like aluminium, manganese or silicon areoften used.

The deoxidizers remove the oxygen from the weld metal.Good ventilation is required when using pure CO2. About 7-12 percent of the CO2 becomes CO (carbon monoxide) inthe arc. The amount increases with the arc length.

A 25% higher current is used with CO2 than with argon orhelium. This causes more agitation of the weld puddle,hence entrapped gases raises to the surface of the weld,so low weld porosity.


Types of shielding gases for GMAWObjectives: At the end of this lesson you shall be able to• state the different types of shielding gases used in Gas Metal Arc Welding (GMAW) process• state the effects of different shielded gases and gas mixtures on ferrous and non-ferrous metals• select the inert gas or gas mixtures for welding different metals using different modes of metal transfer• explain why a gas heater is used in CO2 welding plant.


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Argon carbondioxide: CO2 in argon gas makes themolten metal in the arc crater more fluid. This helps toeliminate undercutting when GMA welding carbon steels.

CO2 also stabilizes the arc, reduces spatter and promotesa straight line (axial) metal transfer through the arc.

Argon-Oxygen: Argon-oxygen gas mixtures are used onlow alloy carbon and stainless steels. A 1-5 percentoxygen mixture will produce beads with wider, less fingershaped, penetration. Oxygen also improves the weldcontour, makes the weld pool more fluid and eliminatesundercutting.

Oxygen seems to stabilize the arc and reduce spatter. Theuse of oxygen will cause the metal surface to oxidiseslightly. This oxidization will generally not reduce the

strength or appearance of the weld to an unacceptablelevel. If more than 2% oxygen is used with low alloy steel,a more expensive electrode wire with additional deoxidisersmust be used.

The desirable rate of gas flow will depend on the type ofelectrode wire, speed and current being used and the metaltransfer mode.

As a rule small weld pools 10 L/min

medium weld pools 15 L/min

and large spray weld pools 20-25 L/min

Too much gas flow can be just as bad as not having enough.The reason being that if the gas flow is too high it willcome out of the MIG Torch.

Suggested gases and gas mixtures for use in GMAW spray transfer

Metal Shielding gas Advantages

Aluminium Argon 0.1 in.(2.5mm) thick; best metal transfer and arc stability; least spatter

75% Helium 1-3 in.(25-76mm) thick; higher heat input than argon25% argon

Copper, Argon Provide good wetting;good control of weld pool for thickness up tonickel 1/8 in.(3.2mm)and alloys

Magnesium Argon Excellent cleaning action

Carbon Steel, Argon Good arc stability; produces amore fluid and controllable weld5-8% CO2 pool; good coalescence and bead contour, minimizes undercutting ;

permits higher speeds compared with argon.

Low alloy Argon Minimizes undercutting; provides good toughnessSteel 2% oxygen

Stainless Argon Good arc stability; produces a more fluid and controllable weld pool,Steel 1% oxygen good coalescence and bead contour, minimizes under cutting on heavier

stainless steelsArgon Provides better arc stability, coalescence and welding speed2% oxygen than 1% oxygen mixture for thinner stainless steel materials

Aluminium Argon and Argon satisfactory on sheet metal argon-helium preferred on thickercopper, argon helium sheet metalmagnesium,nickel andtheir alloys

Carbon steel Argon Less than 1/8 in.(3.2mm) thick; high welding speeds without melt20-25% CO2 through; minimum distortion and spatter; good penetration

CO2 Deeper penetration; faster welding speeds;minimum cost

Stainless 90% helium No effect on corrosion resistance small heat affected zone; noSteel 7.5% argon undercutting; minimum distortion; good arc stability

2.5% CO2



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CO2 gas cylinder and regulator: The shielding gasrequired for GMAW/CO2 welding is supplied from a gascylinder through an outlet valve and regulator.

Gas flow meter: It is a unit which has graduations markedon the glass tube. A flow rate adjustment valve fixed to theflow meter controls the rate of flow of inert gas/CO2 gas tothe welding gun in litre per minute. Fig. 2.

Gas Preheater for CO2 welding (Fig 3): Carbondioxideis filled in cylinders in liquid form. i.e., the CO2 at roomtemperature and high pressure condenses into liquid form.Therefore while welding the liquid CO2 has to be in gaseousform as they enter into the welding torch. CO2 liquid boilsand expands into gas as it passes through the regulator.This causes the gas to cool. If moisture is present in theregulator inlet, it will condense and freeze in the regulator,causing blocking of the gas passage. Therefore to avoidcooling a gas heater is connected to the cylinder toincrease the temperature of the gas leaving the cylinder.Hence a uniform gas flow is maintained during welding.



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Flux Cored Arc Welding (FCAW) Fig.1 is an arc weldingprocess in which the heat for welding is produced by an arcestablished between the flux cored tubular consumableelectrode wire and the workpiece.

There are two major versions of the process, namely selfshielded type (in which the flux performs all the functionsof shielding) and the ‘gas shielded type’, which requiresadditional gas shielding.

The gas shielded type FCAW is widely employed forwelding of carbon steel, low alloy steel and stainless steelin flat, horizontal and overhead positions.

However, the self shielded type FCAW is mainly used forcarbon steel welding and the quality of weld produced bythis type is generally inferior to that of welds made with gasshielded type.

Equipment: The noticeable differences in the equipmentused for GMAW and FCAW, are in the construction ofwelding torch and feed rollers.

The welding torch used for self shielded wire is very simplein construction as there is no need for the gas nozzle.Similarly the feed rollers used for flux cored wires have toensure positive feeding of the wire without applying toomuch pressure on the soft tubular wire.

Metal transfer in FCAW: The metal transfer in FCAWdiffers significantly from GMAW process. FCAW processexhibits two distinctly different modes of metal transfer,namely large droplet transfer and small droplet transfer.However, both are classified as free flight transfer. TheFCAW process does not produce a stable dip transfer asthat of solid wire GMAW. The large droplet transfer occursat the lower current voltage ranges. At higher currentvoltage ranges, the transfer mode changes to smaller

droplet transfer. An important aspect to be observed duringFCAW metal transfer is the presence of the ‘flux pole’ at thecore of the arc column, protruding into the arc. The ‘fluxpole’ appears only during welding with basic type flux coredwire. Fig.2(a) However, with rutile wire ‘flux pole’ does notoccur and the metal transfer is of spray type. Fig.2(b)

Classification of flux cored wires: The basic functionsof the flux contained within the tubular wire include provid-ing protective slag on the weld bead, introducing therequired alloying elements and deoxygenerators into theweld pool and providing stability to the arc, besidesproducing the required shielding medium to protect the arcand weld pool.

Flux cored wires are now available for welding of plaincarbon steel, low alloy steel and stainless steel and alsofor hard facing applications. These wires based on thenature of flux, may be classified as rutile gas shielded,basic gas shielded, metal cored and self shielded.

Rutile gas shielded wires have extremely good arc runningcharacteristics, excellent positional welding capabilitiesand good slag removal and mechanical properties.

Basic gas shielded wires give reasonable arc characteris-tics, excellent tolerance to operating parameters and verygood mechanical properties.

Metal cored wires contain very little mineral flux, the majorconstituent being iron powder and ferro alloys. These wiresgive smooth spray transfer in Argon/CO2 gas mixtures.They generate minimum slag and are suitable for mecha-nised welding applications. Self shielded wires are avail-able for general purpose down hand welding.

The flux cored wires are available in both seamless andfolded types. The seamless type is generally coated withcopper, whereas the folded type wires (i.e. close butt andoverlapped type) are treated with special compounds.


Flux Cored Arc Welding (FCAW)Objectives: At the end of this lesson you shall be able to• explain the flux cored arc welding and narrow gap welding process• explain the type of metal transfer in flux cord Arc welding• classification of flux cored wires


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Deposition rate and efficiency: Deposition rate is de-fined as the weight of metal deposited per unit time. Thedeposition efficiency is defined as the ratio of weight of weldmetal effectively deposited to the weight of wire consumed.

In GMAW welding the deposition efficiency is generallybetween 93% to 97% and in FCAW the correspondingfigure is between 80% to 86%. These values are deter-mined by the spatter losses and slag formation. The lowdeposition efficiency in the case FCAW is due to the slagformation.

Generally the spatter loss can be minimised by usingArgon/CO2 mixed gas instead of CO2 gas.



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Edge preparation of various thickness of metals (GMAW)Objectives: At the end of this lesson you shall be able to• state the edge preparation of GMAW• describe the various types of welding process for required prepartion.

Base metal preparation: For GMAW/CO2 welding theedges and the plate surfaces for welding of ferrous and non-ferrous metals are cleaned similar to Shielded Metal ArcWelding process. The groove angle for single V butt joint incase of CO2 welding is 400 to 450 only when compared to600 used for shielded metal arc welding (Figs 1,2 & 3). Theedge preparation required for the various types of weldingprocess.

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Fig 1




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GMAW defects, causes and remediesObjective: At the end of this lesson you shall be able to• state the weld defect, explain the causes and remedy if the defects.

Excessive spatter

Possible causes Corrective actions

Wire feed speed too high. Select lower wire feed speed.

Voltage too high. Select lower voltage range.

Electrode extension (stickout) too long. Use shorter electrode extension (stickout).

Workpiece dirty. Remove all grease, oil, moisture, rust, paint, undercoating, anddirt from work surface before welding.

Insufficient shielding gas at welding arc. Increase flow of shielding gas at regulator/flowmeter and/orprevent drafts near welding arc.

Dirty welding wire. Use clean, dry welding wire.

Eliminate pickup of oil or lubricant on welding wire from feederor liner.

Excessive Spatter : scattering of molten metalparticles that cool to solid form near weld bead.


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Incomplete fusionIncomplete Fusion — failure of weldmetal to fuse completely with basemetal or a preceeding weld bead.

Porosity

Porosity — small cavities or holes resulting fromgas pockets in weld metal.

Possible causes Corrective actionsInadequate shielding gas coverage. Check for proper gas flow rate.

Remove spatter from gun nozzle.

Check gas hoses for leaks.

Eliminate drafts near welding arc.

Hold gun near bead at end of weld until molten metal solidifies.

Wrong gas. Use welding grade shielding gas; change to different gas.

Dirty welding wire. Use clean, dry welding wire.

Eliminate pick up of oil or lubricant on welding wire from feeder or liner.

Workpiece dirty. Remove all grease, oil, moisture, rust, paint, coatings, and dirt from worksurface before welding.

Use a more highly deoxidizing welding wire.

Welding wire extends too far out of Be sure welding wire extends not more than (13 mm) beyond nozzle.nozzle.

Possible causes Corrective actionsWorkpiece dirty. Remove all grease, oil, moisture, rust, paint, coatings, and dirt from work

surface before welding.

Insufficient heat input. Select higher voltage range and/or adjust wire feed speed.

Improper welding technique. Place stringer bead in proper location(s) at joint during welding.

Adjust work angle or widen groove to access bottom during welding.

Momentarily hold arc on groove side walls when using weaving technique.

Keep arc on leading edge of weld puddle.

Use correct gun angle of 0 to 15 degrees.



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Excessive penetrationExcessive Penetration — weld metalmelting through base metal andhanging underneath weld.

Lack of penetrationLack of Penetration — shallowfusion between weld metal andbase metal.

Possible causes Corrective actionsExcessive heat input. Select lower voltage range and reduce wire feed speed.

Increase travel speed.

Possible causes Corrective actionsImproper joint preparation. Material too thick. Joint preparation and design must provide access to

bottom of groove while maintaining proper welding wire extension and arccharacteristics.

Improper weld technique. Maintain normal gun angle of 0 to 15 degrees to achieve maximum pene-tration.

Keep arc on leading edge of weld puddle.

Be sure welding wire extends not more than (13 mm) beyond nozzle.

Insufficient heat input. Select higher wire feed speed and/or select higher voltage range.

Reduce travel speed.

Burn throughBurn-Through — weld metal meltingcompletely through base metalresulting in holes where no metal remains.

Possible causes Corrective actions

Excessive heat input. Select lower voltage range and reduce wire feed speed.

Increase and/or maintain steady travel speed.



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Waviness of beadWaviness of Bead — weld metal thatis not parallel and does not coverjoint formed by base metal.

Possible causes Corrective actionsUnsteady hand. Support hand on solid surface or use two hands.

DistortionDistortion — contraction of weld metalduring welding that forces basemetal to move.

Possible causes Corrective actionsExcessive heat input. Use restraint (clamp) to hold base metal in position.

Make tack welds along joint before starting welding operation.

Select lower voltage range and/or reduce wire feed speed.

Increase travel speed.

Weld in small segments and allow cooling between welds.



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Heat in put and techniquesObjective: At the end of this lesson you shall be able to• describe the effects of heat on the properties of metals.

Weldments, preheating, heat affected zone, interpasstemperature.

Introduction: During welding, the parent metal is heatedto melting point and after that it is allowed to cool rapidly.The adjacent portion to the welded zone is also heated byto a lower temperature. This causes certain phasetransformations and on rapid cooling, due to heat transferthrough the colder portion of parent metal and atmosphere,the materials hardness and hence mechanical propertiesare also affected.

The width of parent metal that is affected due to the abovecycle is called 'Heat Affected Zone'. It is quite clear thatthe hardness depends on rate of cooling. Higher the coolinghigher will be the hardness. In order to control the coolingrate pre-heating and interpass temperature controls areadopted.

In order to relieve the welding induced stresses and toachieve better metallurgical structure to meet serviceconditions, post - weld heat treatment is followed.

Heat input: The energy supplied by the welding arc in afusion welding process is called arc energy and iscalculated from current voltage and welding speed. Howeverall the arc energy is not utilized for welding; some of it isinvariably lost as shown in Fig.1

The heat input of a single pass weld is calculated bymultiplying the efficiency of the welding process and arcenergy. Therefore heat input at best can serve as a roughguide to the amount of heat supplied to the workpiece.

Temperature changes in welding: Heat moves from onearea to another whenever there is a difference intemperature. Just as water flows downhill, so that flowsdown the temperature hill, warning cold objects at theexpense of warmer ones.

When the source is moved away, the heat in the weld isconducted outward into the plate. The temperature of theweld has fallen, while the plate temperature near the weldis rising.

The weld has cooled still further and the plate temperatureis still rising. The metal reaches a maximum temperatureless than the melting point of the weld metal, and coolingsets in.

Heat Affected Zone (HAZ): The energy applied to createa weld joint is dissipated by conduction to the base metal,welding fixtures and the environment. That part of the basemetal experiencing various thermal cycles is called theheat affected zone (HAZ)

During welding, the HAZ does not undergo welding butexperiences complex thermal and stress alterations. Theimposition of welding thermal cycles on the base materialcauses in the properties of the HAZ.

A welding thermal cycle is characterized by heating rate,peak temperature and cooling rate. Thermal cycles arealso affected by heat input, preheating temperature, platethickness and joint geometry.

Weld joint: A weld joint consists of several zones.

1 Weld metal or mixed zone which is essentially a solifiedstructure.

2 Unmixed zone in the base metal adjacent to the fusionline where the base metal has melted but is not mixedwith the filler material.

3 Partially melted zone which has been thermal cycleswith peak temperatures and,

4 Heat affected zone which has not melted but is exposedto thermal cycles with temperature less than the solidstemperature.

Each zone because of its characteristic micro structuralfeatures has different properties.

Heat affected zone microstructure: The relevant portionof the iron-carbon phase diagram along with a schematicsketch of a weld and HAZ is shown in Fig.2

The extent of energy loss varies with the welding process,welding parameters, type of material, preheat temperatureetc. To account for the energy loss and estimate the actualenergy given to the workpiece, a term known as heat inputis employed.

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41Fabrication: Welder (NSQF - 4) - Related Theory for Exercise: 2.2.80

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The region of the HAZ where extensive grain growth of theweld metal takes place is referred to as coarse grainedHAZ (CGHAZ) 1300°C.

The region of the HAZ next to it,where peak temperatureis in the range of 900-1200°C and austenite grain sizeremains small, is called fine grained HAZ (FGHAZ).

CGHAZ is having maximum hardness and poor toughnessproperties compared to the rest of the HAZ. So the preheattemperature used to reduce the cooling rate.

Heat affected zone and how to avoid risk of cracking

The region of the parent metal, which undergoes ametallurgical change as a result of the thermal cycle iscalled heat affected zone. A typical HAZ is shown if Fig 3.

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If the carbon equivalent (CE) exceeds 0.4,the weldingsituation changes due to the possibility of cracking in theheat affected zone and due to increase in volume ofmartensite, cracks will usually develop the phenomenoncalled underbead cracking.

The normal structural steel has a hardness of 190-200BHN. In HAZ,depending upon thickness,carboncontent,hardness of 350-450 BHN may be reached. Thelevel of hardness depends upon cooling rate. The risk ofcracking is higher when hardness exceeds a certain levelcorresponding to higher rate of cooling.

The interaction of cooling rate and carbon equivalent isillustrated in Fig 4. At low levels of carbon equivalent fastrates can be tolerated before there is risk of cracking;except in thick section, HAZ cracking is rarely experiencedwith CE values below 0.39%. At high levels of CE,sayaround 0.48%,there is high risk of cracking even at slowercooling rates.

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However, Appropriate preheating of the parent metal andor low levels of hydrogen in the weld metal can eliminatethis problem.

Higher level of hydrogen is harmful. Hydrogen is absorbedin the molten weld pool from a variety of sources, moisturein the flux covering of an electrode or in the shielding gas,grease on the joint faces and so on. Hydrogen can flow(diffuse) readily through hot steel and pass from the weldpool in to the HAZ causing a major risk of cracking.

Gas shielded processes such as MAG and TIG areinherently low in hydrogen with levels 5-10 ml/100 gramand are thus effective in avoiding cracking.

Heat input and the thickness of the metal in the joint affectthe cooling rate in the unit.

In thick sections cooling rate is faster than in thin.Preheating temperature slows down the cooling ratethrough the temperature range within which a hardenedstructure is formed i.e., 300-200°C. Preheat also helps toreduce the risk of cracking by allowing any hydrogen inthe heat affected zone to flow into the parent metal wherehardening has not taken place.

The interdependence of principal factors i.e., CE, coolingrate (heat input, joint type and thickness), hydrogen conentand preheat (temperature of parent metal during welding)governing the risk of HAZ cracking is complex.

The problem of underbead cracking can eaily be overcomeby preheating the weld joint just prior to welding or bychoosing a proper low hydrogen electrode.


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Purpose of preheat: There are four reasons why pre-heat is useful in weld fabrication. They are

a The use of preheat lowers the cooling rates in the weldmetal and in the heat affected zone. This results in amore ductile metallurgical structure, one that will re-sist weld cracking.

The slower the cooling rate permits hydrogen to diffuseout harmlessly, without causing cracking.

Preheat reduces shrinkage.

It also brings same steels above the temperature wherebrittle fracture might occur during welding.

No steel is immune to hydrogen-induced cracking. Addi-tionally, preheat can be used to help ensure specificmechanical properties, such as notch toughness.



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Heat distribution and effects of faster coolingObjective: At the end of this lesson you shall be able to• explain the necessists of heat distribution in welding.

Heat input is increased with increasing wire feeding speedbut increasing welding speed decreases the welding heatinput. When heat input increases, the cooling rate decreasesfor weld metal and increases the volume fraction of temperedmartansite and coarsening of the microstructure of weldzone.

The outcome of microstructural examination and mechanicaltests should that fast cooling, immediately after submergedwelding can reduce the width of heat affected and coarsegrained zones, as well as improving the low temperatureimpact toughness.

Heat input is increased with increasing wire feeding speedbut increasing welding speed decreases the welding heatinput. When heat input increases, the cooling rates decresesfor weld metal and increases the volume of fraction oftempered, marking and coarsening of the microstructure ofweld zone.


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Table 1Preheating of various metals

Metal Temperature °CNickel alloys (wrought) Warm it below 16°Nickel alloys (cast) 90° - 200°Copper and copper alloys 200° maximumSilicon bronze 90°Brass low zinc 200° - 260°Brass high zinc 260° - 370°Phosphor bronze 150° - 200°

The preheating reduces the rate of cooling after welding.This is necessary to prevent the weld metal from crackingin restrained/rigid joints. Also some of the non-ferrousmetals like copper, brass, aluminium, etc. expand moredue to heating and ferrous metals like cast iron, mediumand high carbon steels require preheating as they are toobrittle. These materials are necessarily to be preheated toavoid cracking or distortion. In some cases, it is alsonecessary to preheat during welding between each layerof deposition.

The minimum preheating temperature for satisfactorywelds of different grades of steel, cast iron, non-ferrousmetals will depend upon the: (Fig 1)

– type of metal

– composition and properties of the parent metal

– thickness of the plate

– type of joint

– degree of restraint of the joint

– rate of heat input.Do not allow the temperature to drop below theminimum preheating temperature between eachweld run.

Fabrication Related Theory for Exercise 2.2.82 & 2.2.83Welder - Gas metal arc welding

Preheating and post heating treatmentObjectives: At the end of this lesson you shall be able to• explain the necessity of heat treatment in welding• describe different methods of heat treatment applied in welding• state the purpose of preheating• state the purpose of post heating.

Different methods of heat treatmentDirect preheating, Indirect preheating, Local preheating

Preheating and its purpose: Preheating means heatinga joint to be welded before or during welding to a certaintemperature as shown in tables 1 and 2.


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The preheating temperature can be checked by tempera-ture indicating crayons. (Fig 2)

If the job and area to be preheated are large, then it is donein a preheating furnace (Fig 3).

If it is small localised preheating is applied to the joint areaonly. This is called local preheating. (Fig 4)

The important aspects to be considered while post–heating are:

– the rate of heating– temperature to which the part is to be post-heated– holding time in the furnace– the rate of cooling.

Post heating of carbon steels depends on the thickness ofthe base metal and its carbon content. (Fig.5)

Post heating retards the rate of cooling of awelded joint.

For plain carbon steels the joint is heated from 100°C to300° C for general post heating. This treatment will reducethe cracking tendency of carbon steel and cast iron. If theyare not post heated, cracks may develop.

Also the welding heat can develop hardness and brittle-ness in some areas of the joint. In addition the grains of thebase metal in the heat affected zone and fusion zone willgrow in size which will change the property of the weldedjoint.

In the case of joints which are not free to expand i.e.,restrained joints and in joints in which there is a stressalready present before welding, the residual stresses willbe more after cooling of the joint. If these residual stressesare not removed after welding, then the joint will fail ordistort when they are put into use or the joint is machinedor the joint is subjected to dynamic loading.

To avoid the above problems a welded job is usually eithernormalised or annealed or stress-relieved.

Post heating: Post heating means that the part is heatedimmediately after welding. The reasons for post heating areto prevent hard and brittle spots from forming in theweldment. It also relieves the residual stresses caused bythe welding heat and due to welding of a rigid joint.



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TABLE 2

Preheating temperatures for plain and alloy steels Approximate composition Recommended

(percentage) preheatingtemperature

% Carbon °C °FPLAIN CARBON STEELS C

Plain carbon Below 0.20 Up to 95 Up to 200Plain carbon 0.20 to 0.30 95 to 150 200 to 300Plain carbon 0.30 to 0.45 150 to 280 300 to 500Plain carbon 0.45 to 0.80 260 to 425 500 to 800

CARBON MOLYBDENUM C MoSTEELS

Carbon molybdenum 0.10 to 0 .20 0.50 150 to 260 300 to 500Carbon molybdenum 0.20 to 0.30 0.50 200 to 315 400 to 600Carbon molybdenum 0.30 to 0.35 0.50 260 to 425 500 to 800

MANGANESE STEELS C Mn Si

Silicon structural 0.35 0.80 0.25 150 to 260 300 to 500Medium manganese 0.20 to0 .25 1.0 to 1.75 150 to 260 300 to 500SAE T 1330 0.30 1.75 200 to 425 400 to 600SAE T 1340 0.40 1.75 260 to 425 500 to 600SAE T 1350 0.50 1.75 315 to 480 600 to 90012% Manganese 1.25 12.0 Usually not Usually not

required required

NICKEL STEELS C Ni

SAE 2015 0.10 to 0.20 0.50 Up to 150 Up to 300SAE 2115 0.10 to 0.20 1.50 95 to 150 200 to 3002”% Nickel 0.10 to 0.20 2.50 95 to 200 200 to 400SAE 2315 0.15 3.50 95 to 260 200 to 500SAE 2320 0.20 3.50 95 to 260 200 to 500SAE 2330 0.30 3.50 150 to 315 300 to 6000SAW 2340 0.40 3.50 200 to 370 400 to 700

LOW CHROMEMOLYBDENUM STEELS C Cr Mo 0 C 0 F

2% Cr. ”% Mo Up to .15 2.0 0.5 200 to 315 400 to 6002% Cr. ”% Mo 0.15 to 0.25 2.0 0.5 260 to 425 500 to 8002% Cr. 1% Mo Up to 0.15 2.0 1.0 260 to 370 500 to 7002% Cr. 1% Mo 0.15 to 0.25 2.0 1.0 315 to 425 600 to 800

MEDIUM CHROMEMOLYBDENUM STEELS C Cr Mo

5% Cr. ”% Mo Up to 0.15 5.0 0.5 260 to 425 500 to 8005% Cr. ”% Mo 0.15 to 0.25 5.0 0.5 315 to 480 600 to 9008% Cr. 1% Mo 0.15 Max. 8.0 1.0 315 to 480 600 to 900

PLAIN HIGHCHROMIUM STEELS C Cr

12 to 14% Cr TYPE 410 0.10 13.0 150 to 260 300 to 50016 to 18% Cr TYPE 430 0.10 17.0 150 to 260 300 to 50023 to 30% Cr TYPE 446 0.10 26.0 150 to 260 300 to 500



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Pre-heat treatment and post weld heat treatmentObjectives: At the end of this lesson you shall be able to• explain heat treatment• explain the different methods of heat treatment.

Heat treatments: Heat treatments are used to obtaincertain desired properties. Essentially, heat-treating met-als consists of heating and cooling it after it has reachedthe solid state. There are a number of different methods ofheat treatment for various steels in today’s industry.

Normalising: Normalising is very similar to annealingexcept that the steel is held above the critical temperaturevery briefly and the cooling takes place in air at normaltemperature. Normalising will result in refining the grainstructure of a metal. It is sometimes used after quenching.

Annealing: Annealing involves heating of metal to atemperature above the critical point and allowing it to coolslowly. The purpose of annealing may be to accomplishone or more of the following.

– To soften the metal, e.g. to improve machinability.

– To relieve internal residual stresses.

– To refine the grains.

– To improve ductility.

– Homogenizing to reduce.

Hardening: Hardening increases the strength of thepieces after they are fabricated. It is accomplished byheating the steel to a temperature above the critical point,and then cooling it rapidly in oil, water or lime. Onlymedium, high and very high carbon steels can be hardenedby this method. The temperature at which the steel mustbe heated varies with the steel used.

Case hardening: It is the process of hardening the outersurface of steel. It is done by inducing additional carboninto the case of the steel. This is done in a number ofdifferent ways all of which require heating to a hightemperature and rapid cooling.

Some of the methods employed are:

– to pack the steel part in a sealed metal box whichcontains some carbonizing material

– to immerse the steel part in a molten cyanide salt bath

– to dip the heated steel part in a container havingpowdered cyanide

– to heat the steel part and pass carbonizing gas over it

– to employ manual or machine-controlled oxy-acety-lene flame process.

Tempering: Tempering (grain refining) is used to relievesome of the brittleness which occurs after a piece of steelhas been fully hardened, and to make the steel tough.

It is accomplished by reheating the hardened metal to aspecified temperature, depending upon the hardness to beremoved, and then quenching.

Quenching: Quenching is the rapid cooling of a metalusually done by immersing it in oil or water. This will causecertain changes in the structure of the metal. For example,carbon steel that is quenched will form a martensitestructure.

Stress relieving: Stress relieving is a means of removingthe internal stresses which develop during the weldingoperation.

This process consists of heating the structure to a tem-perature below the critical range (approximately 590°C)and allowing it to cool slowly. Another method of relievingstresses is peening (hammering). However, peening mustbe undertaken with considerable care because there isalways the danger of weakening the physical strength ofthe metal.

Stress relieving should be done only if there is a possibilitythat the structure will crack upon cooling and no othermeans can be used to eliminate the expansion andcontraction forces.

Hardening and temperingObjectives: At the end of this lesson you shall be able to• explain the principle of hardening• describe the effect of carbon content in hardening• explain the process of hardening• explain the tempering of steel• desribe the purpose of tempering• explain the factors on which the mechanical properties of hardened steel depend.Introduction: If a piece of steel is heated to a sufficientlyhigh temperature, all the carbon will be dissolved in thesolid iron to form the solid solution, austenite of the steel.When it is slowly cooled, the change in the arrangement ofthe iron atoms will cause a solid solution called ferrite to beproduced. The solid solution can only contain up to 0.006%

carbon, and so the excess carbon will be forced to leave thesolid solution, and produce cementite. This will, withferrite, form a laminated structure called pearlite.

The principle of hardening: If steel is cooled rapidly(quenched) the excess carbon will not have sufficient timeto leave the solid solution with the result that it will be



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trapped in the iron, and so cause an internal distortion. Thisinternal distortion is the cause for the increase in thehardness of steel with a corresponding reduction in itsstrength and ductility. This is the basis of the hardeningprocess.

The mechanical properties produced as a result of thistreatment will depend upon

– the carbon content of the steel

– the temperature to which it is heated

– the duration of heating

– the temperature of the steel at the start of quenching

– the cooling rate produced by quenching.

The effect of carbon content upon the hardness producedby the process is illustrated in Fig 1.

Fig 2 illustrates the temperatures to which steels areheated before quenching.

Cooling: Then the steel is cooled in a suitable quenchingmedium at a certain minimum rate called the criticalcooling rate. The critical cooling rate depends upon thecomposition of the steel. This cooling transforms all theaustenite into a fine, needle-like structure called martensite,the appearance of which is shown in Fig 3.

The structure of steel treated this way is very hard andstrong, but very brittle.

The quenching medium: The quenching medium con-trols the rate of cooling.

For rapid quenching a solution of salt or caustic soda inwater is used.

For very slow quenching a blast of air is sufficient.

Oil gives an intermediate quenching.

Water and oil are the most common quenching mediaused.

Air quenching is suitable only for certain special alloysteels.

Tempering: After hardening, steel is usually reheated toa suitable temperature below the lower critical point (heat-ing ) to improve its toughness and ductility but it is done atthe expense of hardness and strength. It is done in orderto make the steel more suitable for service requirements.

Purpose of tempering the steel: Steel, in its hardenedcondition, is generally too brittle and too severely strained.In this condition, steel cannot be used, and hence it has tobe tempered.

The aims of tempering are:

– to relieve the steel from internal stresses and strains– to regulate the hardness and toughness– to reduce the brittleness– to restore some ductility– to reduce shock resistance.

Process of tempering: The tempering temperature de-pends upon the properties required, but it is between 180°Cand 650°C. (Fig 4) The duration of heating depends uponthe thickness of the material. Tools are usually temperedat a low temperature. The temperature itself is judged bythe colour of the oxide film produced upon heating.(Table 1)

Soaking time: After heating, the steel is held at thattemperature for some time. Normally 5 mts are allowed assoaking time for 10 mm thickness of steel.



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Table 1Tempering temperature

Temper colour Temperature in °CPale straw 230

Dark straw 240

Brown 250

Brownish purple 260

Purple 270

Dark purple 280

Blue 300

In a manufacturing plant, when heat-treating is done on aproduction basis, modern methods are used. Temperingis done in controlled atmosphere furnaces with the tem-peratures controlled by modern instruments. Under suchconditions, it is possible to obtain accurate and uniformresults in any number of pieces.

This method is not, however, suitable for accurate tempera-ture assessment.

Fig 5 illustrates the appearance of the microstructure ofhardened and tempered steel.

Generally, tempering in the lower temperature range for anincreased time provides greater control in securing thedesirable mechanical properties. Such heat treatmentmay not be feasible under all conditions. For precisionwork, where results justify the method, and for certaincombination of mechanical properties, tempering for longperiods of time in a lower temperature range provides areliable method of getting the desired results.

Fig 6 illustrates how the mechanical properties of hardenedsteel can be modified by tempering.



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Annealing temperature

Carbon Temperature°Ccontent %<0.12 875 to 9250.12 to 0.25 840 to 9700.25 to 0.50 815 to 8400.50 to 0.90 780 to 8100.90 to 1.3 760 to 780

Purpose of annealingAnnealing is done:

– to obtain softness

– to improve machinability

– to increase ductility

– to relieve internal stresses

– to reduce or eliminate structural inhomogeneity

– to refine grain size

– to prepare the steel for subsequent heat treatmentprocess.

Normalising: Due to continuous hammering or unevencooling, strains and stresses are formed in the internalstructure of steel. These should be removed from forgingsor castings; otherwise, they may fail at any time while inuse.

Normalising is done to produce a fine grain for uniformity ofstructure and for improved mechanical properties.

The normalising process: In this process, steel is heatedto a suitable temperature depending upon its carboncontent, (Fig 2) and held at the same temperature, andthen, cooled freely in air.

The properties of steel depend upon its composition andits structure. The properties of steel and its structure canbe changed by heating it to a particular temperature andthen, allowing it to cool at a definite rate. The process ofheating and cooling for changing the structure of steel, andthus obtaining the required properties is called ‘Heattreatment of steel’.

If the steel is heated to a suitable temperature, and then,slowly cooled, the steel will be soft, weak and ductile.

If the steel is heated to a suitable temperature, and then,rapidly cooled (quenched) the steel will be hard and brittle.

Classification of heat treatmentsThe treatments that produce equilibrium conditions areannealing and normalising.

Treatments that produce non-equilibrium conditions arehardening and tempering (usually done in conjunction witheach other).

Annealing: In this process, steel is heated to a suitabletemperature depending upon its carbon content (Fig 1),and is held at that temperature for sufficient time, and thenslowly cooled to room temperature.

Heat treatment of steels (Annealing and normalising)Objectives: At the end of this lesson you shall be able to• explain the purpose of heat treatment• explain the purpose of annealing• distinguish between the processes of annealing and normalising.

The heating, soaking (holding the temperature) and slowcooling cause the grains to become large, and so, producesoftness and ductility. For annealing, hypoeutectoid steelis heated to 30° to 50°C above the upper critical tempera-ture, and it is 50°C above the lower critical temperature forhypereutectoid steel. (Fig 1)

Soaking time at this temperature is 5 mts/10 mm ofthickness for carbon steel.

The cooling rate for carbon steel is 100 to 150°C/hour.

The cooling is done in the furnace itself by switching off thefurnace or the steel is covered either with sand or dry limeand dry ash.



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Normalising is usually done before machining and beforehardening to put the steel in the best condition for theseoperations.

The steel is heated to a temperature (300 to 400C above theupper-critical temperature) at which all austenite is presenteven in the case of high carbon steel. This is because thisprocess is the first step towards providing the final properities,and it is necessary to start with austenite to ensureuniformly.

The heated piece for normalising should not bekept at a wet place, in wet air or kept in forcedair as they will induce some hardness.

Use of temperature indicating crayonsThe temperature of the preheated job can be checked bywax crayons. Marks are made on the cold job pieces bythese crayons before preheating and after the job piecesreach the preheating temperature the marks will disappear.

This indicates that the job has been heated to the requiredpreheating temperature. Different wax crayons are avail-able for checking different temperatures. The temperaturewhich is checked by the crayon will be marked on it.



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Principles of submerged arc welding: Submerged arcwelding is an arc welding process that uses an arc betweena bare metal electrode and the weld pool. The arc and themolten metal are hidden by a blanket of granular flux on theworkpieces. (Fig 1)

For plate thicknesses higher than 25mm a double Vee orsingle U or double "U" edge preparation is done Fig.3shows fillet welds done by submerged arc welding.

The "T" and Lap joints shown in Fig.3 are tilted to 450 to weldthem in flat position. If the thickness of plates are more than16mm in T fillet joint then the edge of the vertical plate isbevelled by 450 and the joint is welded without a root gap.

Types of submerged arc welding processThere are two types of SAW.

– Automatic

– Semi-automatic

Automatic SAW: In this type the arc voltage, arc length,speed of travel and electrode feed are automatically con-trolled.

Semi-automatic SAW: The arc length, flux feeding andelectrode feed are automatic but the speed of travel iscontrolled by the operator.

Metals which can be welded by SAW: In submerged arcwelding, low and medium carbon steels, low alloy steels,high strength steels, quenched and tempered steel andstainless steel can be welded.

Metals weldable by saw

Base metal WeldabilityWrought iron WeldableLow carbon steel WeldableLow alloy steel WeldableHigh and medium carbon Possible but not popularHigh alloy steel Possible but not popularStainless steel Weldable

Edge preparation in SAW process: The edge prepara-tion for Butt welds are as shown in Fig.2.


Submerged arc welding process principlesObjectives: At the end of this lesson you shall be able to• explain the principles and applications of submerged arc welding• explain the SAW• describe the welding procedure of the above processes• state the advantages and limitations of the above processes.


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Parts of a SAW machine and their functions (Fig 4)

A welding power source to supply welding current to theelectrode at the contact tube.

Arrangement for holding the flux and feeding it on the headof the arc.

A means of traversing the joint.

Fluxes: Fluxes used with submerged arc welding aregranulated fusible mineral materials which are free fromsubstances capable of producing large amount of gasduring welding.

Flux when cool is non-conductive, but when molten it ishighly conductive and allows high current.

The flux protects the weld pool from atmospheric contami-nation and influences deep penetration.

Electrode: Bare or lightly copper coated rods or wires areused as electrodes in SAW. These electrodes areavailable in coil or reel form.

Standard reels with diameters 2 to 8 mm are available.

Welding procedure (for striking the arc): The electrodemomentarly contacts the work and is withdrawn slightly.

Arc start: Arc starting is difficult in submerged arc weldingbecause of the flux cover. It is important to start the weldat a specific point on the joint.

Method of starting arc by using steel wool or ironpowder: A rolled ball of steel wool 10 mm in dia. is placedat the required spot on the joint and the electrode wire islowered on to it till it is lightly compressed. The flux is thenapplied and when the welding is commenced the steelwool or iron powder conducts the current from the wire tothe workpiece, while at the same time it melts away rapidlyas the arc is formed.

Clean the prepared workpiece and place it in position withprovision for backing up. Fill the hopper with flux and insertthe electrode ends into the welding head.

Adjust the voltage, the current and the welding speed asindicated in Table 1 and 2.

Start welding by striking an arc beneath the flux on thework.

The entire welding zone is buried under a blanket of flux andlongitudinaly it travels along the seam.

Use ‘run on’ and ‘ run off ‘ pieces for starting and ending toavoid formation of crater and beginning and ending faults.(Fig 5)

Advantages of SAW– High quality weld metal– High deposition rate and speed– Smooth, uniform finished weld– No spatter– Little or no smoke– No arc flash– High utilization of electrode wire– No need for protective clothing

A wire feeder to drive the electrode to the work through thecontact tube of the welding gun or welding head.



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Table 1Submerged arc welding parameters for single electrode

(For fillet welds by automatic welding)

Limitations: The submerged arc welding process is lim-ited to flat position and horizontal fillet position.

Table 2Submerged arc welding parameters for single electrode

(For Butt welds by Automatic welding)

Weld size 5 6 8 10 12 16 20(mm)Plate 6 8 10 12 16 20 25thickness(mm)Electrode 3.2 4 5 5 5 5 5size (mm)Current(amp) DC 520 620 720 800 870 920 970Volts 30 32 34 36 38 39 40Welding 1.4 1 0.7 0.56 0.36 0.25 0.20speed(m/min.)Electrode 0.10 0.18 0.28 0.40 0.70 1.1 1.6req'd (Kg/m)Flux req'd 0.05 - 0.75 - 0.14 - 0.18 - 0.33 - 0.53 - 0.83 -(Kg/m) 0.09 0.12 0.18 0.27 0.45 0.75 1.2Total time 0.012 0.016 0.0024 0.03 0.047 0.67 0.09(hr/m of weld)



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Fabrication Related Theory for Exercise 2.2.85Welder - Gas metal arc weldingElectro slag welding and electro gas weldingObjectives: At the end of this lesson you shall be able to• state the principle of electro slag welding & electro gas welding• explain the procedure• state the advantages of electro slag welding• state application and limitations.

IntroductionElectro slag welding process is considered as a furtherdevelopment of submerged arc welding and developed forthe welding of thick plates with a single pass. With thisprocess, plates of 25mm or more can be joined withoutmultiple passes and without edge preparations such asbevel joints, 'V' joints and 'J' joints and 'U' joints and re-duced the twice in the fabrication.

Principle: The electro-slag process is fully automatic.The plates to be joined are kept in vertical position. (Theaxis of the weld joint is vertical) The flux is continuously fedinto the area being welded where it forms a cover of moltenslag over the weld metal. This covering of molten slagserves as a major source of heat for the electro-slagwelding process. The welding takes place in the flat ordown hand position with vertical upward travel of the weldpool. (Fig 1)

Operation: An electric arc is struck between the electrodeand the joint bottom with the help of a piece of steel wool.

Welding flux is added which melts by the heat of the arc.When a thick layer of hot flux or molten slag is formed, thearc action stops and the electric current passes from theelectrode to the workpieces through the conductive slagpool.

The conductive slag pool remains molten because of itsresistance to the electric current passing between theelectrode and the work through it. The temperature of thismolten slag pool is 1650°C at the surface and 1930°Cinside under the surface. The heat fuses the edges of theworkpiece and the welding electrode. Fig 2 shows anelectro-slag welding process in progress.

The function of welding flux: The welding flux shieldsthe molten metal and reduces oxidation. It cleans impuri-ties from the molten metal. The flux is added in smallquantities periodically.

Copper dams or shoesWater-cooled copper shoes contain and confine the mol-ten slag and molten welding metal until it solidifies. Thecopper shoes accelerate solidification of the weld metal.

Advantages– Less edge preparation.

– Thicker steels can be welded in a single pass.

– High deposition rate.

– Residual stresses and distortion low.

– Less flux consumption.

– No spatter.


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Application– Forging and castings of heavy thickness can be welded.

– Some alloys such as low carbon and medium carbonsteels can be welded.

– High strength structural steels can be welded.

– High strength alloy steel such as stainless steel andnickel alloy can be welded.

Limitations: This process can be used only for welding ofvery heavy plates.

Equipments1 AC Power Source

2 Wire Feeder

3 Electrode Guide Tube

4 Control Panel

5 Travel Carriage

6 Water cooled Copper Shoes

1 AC power sourceElectro Slag Welding requires a Power Source, capable ofdelivering the AC welding current, recommended for theprocess. The Power Source is stationary.

2 Wire feederThe function of the wire feeder is to deliver the electrode wirethrough the guide tube to the molten flux.

3 Electrode guide tubeThe tube is insulated materials to avoid short circuiting. Itis made of beryllium copper alloys because this alloy canwithstand high temperature and retains its strength.

4 Control panelIt consists a mounting near the welding head which as thefollowing components

a Ammeter, Voltmeter and a remote control for each ofthe Power Source.

b A speed control for each wire drive motors.

c Alarm Systems for equipment mall functions.

5 Travel carriageAll functions of the machine are controlled from a Panellocated at the vertical transport carriage assembly.

6 Water cooled copper shoesThe Copper Shoes accelerate solidification of the weldmetal. The functions of the Copper Shoes are to maintainthe molten metal and molten slag until it solidifies.

Electro gas weldingElectro Gas Welding is an automatic welding Process thatuses a Carbon Di-oxide (CO2) or a mixture of CO2 and ArgonShielding Gas to protect the Arc and molten weld metalfrom Atmospheric Contaminants. A Consumable Fluxcored or Solid electrode wire is continuously andmechanically led into the arc and molten pool. EdgePreparation is not necessary. This process may be carriedout by two methods

1 Solid Electrode Process

2 Flux Cored Electrode Process

Solid electrode processIn this Process, generally single electrode is fed into thejoint. Two electrodes can be used for welding of thickplates. The Shield is done by the CO2 gas and no extra fluxis required.

Flux cored electrode processIn this Process, electrode with flux is fed into the joint anda thin layer of Slag on the top of molten pool. The Shieldingis done by Flux.

Electro gas welding equipment and suppliesThe Equipments of Electro Gas Welding are similar toElectro Slag Welding but the difference is Shielding Gas.

1 AC power source

2 Water cooled copper shoes

3 Welding gun

4 Wire feeder

5 Shielding gas and

6 Control panel

ApplicationsIt is used for welding of thick plates in vertical position. Itcan be applied on Carbon Steel and Alloy Steels.

Advantages1 Welder can see the welding pool easily.

2 Restarting the weld is easier.

DisadvantagesIt is not suitable economical for welding metal lesser than25 mm. thick.



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Thermit weldingObjectives: At the end of this lesson you shall be able to• state the principle of thermit welding• describe the parts of thermit welding equipment• explain the sequence of operation for completing the weld• state the application of thermit welding.Thermit weldingThermit is a trade name for a mixture of finely divided metaloxide (usually iron oxide) and a metal reducing agent.(almost always aluminum). The thermit mixture may con-sist of about five parts of aluminium and eight parts of ironoxide, and the weight of thermit used will depend on thesize of the parts of to be welded. The ignition powder usu-ally consists of powdered magnesium or a mixture of Alu-minum and Barium Peroxide.

Principle of thermit weldingThe heat necessary for joining in the thermit welding pro-cess is obtained from a chemical reaction that takes placebetween a metal oxide (Iron oxide) and a metal reducingagent. (Aluminum) When ignited by using a burning mag-nesium ribbon in one spot of thermit mixture. The reactionspreads throughout the mixture. The tremendous heat re-leased approximately 2760°C (5000°F) causes the iron tochange to a liquid state within 25 to 30 seconds. As thealuminium in the mixture combines with the oxygen fromthe iron oxide, it forms Alumina oxide, which serves asslag and float to the top. Thermit reaction is an exother-mic process. There are two types of Thermit Welding:

1 Plastic or Pressure Thermit Welding

2 Fusion of Non-pressure Thermit Welding

Plastic or Pressure thermit weldingThis type is used mainly for butt welding of thick pipes ofrails. In this weld, pressure is used to join the metal. Thework pieces are clamped into C.I moulds and are forcedtogether when the desired temperature is achieved. Thethermit is heated in a crucible above the work. While heat-ing, the lighter aluminium oxide slag rises to the top .When pouring temperature is reached, the thermit solu-tion is poured into the mould. When the thermit mixturehas heated the work pieces sufficiently, the work piecesare forced together forming a pressure butt weld. The wholewelding process takes 45 to 90 seconds for welding ofthick walled pipe.

Fusion of non-pressure thermit weldingIn this process, the work pieces are lined up, leaving aspace between the ends tha are to be welded. Wax isplaced between the joint. The whole frame is suspendedin a mould, and then the molten metal is poured. The firststep in preparing a non-pressure thermit weld is the clean-ing of the joint.

Equipments, materials and suppliesThe thermit welding processs requires an adequate sup-ply of

1 Thermit mixture

2 Thermit Ignition powder and a

3 Device (Flint Gun, Hot Iron Rod etc…)

Thermit mixtureThe most commonly used types of thermit for welding thevarious ferrous metals are:

1 Plain Thermit

2 MS Thermit or Forging Thermit

3 Cast Iron Thermit

4 Steel Mill Wabblers

5 Rail welding Thermit

6 Thermit for welding electric connections

Plain thermitA mixture of finely divided Aluminium and Iron Oxide. Thisis the basic of most thermit mixtures and yields maxi-mum temperature.

MS thermitThis is a plain thermit with the addition of manganese andmild steel punching, and is used in welding steel. Manga-nese is added to adjust the chemistry of thermit mixture(Carbon may also be added, the mild steel punching areused to augment the metal content)

Cast iron thermitCast iron Thermit consists of plain thermit with addition ofFerro-silicon and Mild Steel punching and is used for weld-ing cast iron. The mild steel punching augments the totalmetal content. Unless the weld area is post heat treated,this weld metal is generally not machinable and, becauseof the different contraction between it and the cast ironparent metal. It is limited to use where the maximum welddimension is less than 8 times its width.

Thermit for steel mill wabblersThis consists of plain thermit with additions of manga-nese and carbon to provide a hard, wear resistant, ma-chinable steel especially adopted to the building up ofworn wabbler ends of steel mill rolls.


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Thermit for welding railThese mixtures usually consist of plain thermit with addi-tions of carbon and manganese to adjust hardness of thedeposited metal to the hardness of the rail being welded.Alloys are also added for controlling resistance to abra-sion and to act as grain refiners.

Thermit for welding electric connections

This consists of Copper Oxide and Aluminium.

Thermit ignition powderThere are a number of different ignition powders available.Barium per oxide is frequently used. The thermit ignitionpowder is ignited directly with a match. This should bedone by partly burying the match head in the mixture andigniting the match with a thin red hot rod. This avoids thedanger of burns injuries to the finger or hand due to thesudden flare in igniting. The thermit ignition powder canalso be ignited with a spark from a flint gun or by using aburning magnesium ribbon.

Thermit welding procedureThe ends are to be welded are thoroughly cleaned of scaleand rust. After cleaning, the parts to be welded are to belined up with a gap of 1.5 to 6mm depending upon the sizeof parts. This gap compensates the contraction of thermitsteel and the shrinkage of the base metal. The next stageis making wax pattern of the weld. A refractory sand mouldis rammed up around the wax joint and necessary gatesand risers provided. Ramming should be light between themoulding sand and the wax. When ramming is completed,the pattern may be drawn and loose sand may be wipedout. Then, the heat is given to the wax pattern through theheating gate to melt and burn out wax. The heating iscontinued until the ends to be welded are at a red heat.This prevents the thermit steel being chilled, as it wouldbe if it came into contact with cold metal. The preheatinggate is now sealed off with sand. Now, charge the thermitin the crucible. The approximate weight of thermit is 12 to14 kgs against one kg. of wax. The outside shell of cru-cible is made by steel and is lined with manganese tarlining. At the bottom, a magnesia stone and a thimble isprovided through which the tapping pin works. The thimbleis inserted in the stone which provides a channel throughwhich the molten metal is poured for each reaction a newthimble is used. The thimble is plugged by suspendingthe tapping pin and placing a metal disc above pin. Themetal disc is lined with refractory sand. At the top of thethermit, low ignition temperature thermit is placed in thecrucible. When ignited in one spot of thermit mixture, the

ApplicationThermit welding is mainly used in rail welding, concretereinforcement rod welding, building up of steel mill wobblerends and for electrical connections.

1 Rail weldingRails are welded to create long railways for electrifiedand other fast tracks. This increases the passengercomfort and keeps the maintenance cost down. Spill-age of goods (like coal) is minimised in mining industry.

2 Reinforcement steel rod weldingIn big building projects, a massive number of joints areto be carried out for joining of reinforcement steel at ashort time. Thermit welding is applied by using prefab-ricated moulds with reaction chambers.


reaction spreads throughout the mixture. The reacted canbe heard, as soon as the noise of reaction stops, thecrucible should be tapped. The intense heat of thermitmelts the preheated ends of the parts to be welded andthe fusion welding takes place. Then the mould is allowedto cool overnight. If possible or at least for 12 hours andcut away the gates and risers with a cutting torch andfinish the weld. (Fig 1)


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Backing strips and backing barsObjective: At the end of this lesson you shall be able to• understand the principle of backing strips and backing bars.

DefinitionWhile welding the product supported and control the dis-tortion of the concened jobs/product. To minimise the dis-tortion and contraction we can use backing strips andbacking bars.

The following sketchs are to be used.

The temperature and fast cooling is applied on fully heattreated samples.

The microstructure results due to cooling at the maxi-mum holding temperature as well as independant of theapplied pressure value.

The effect of heat treatment and cooling rate on the prop-erties of fast cooling from the upper limit and proesizedistruction do not change significantly throughout thesample sarees.

In the experiments the effect of fast and slow cooling thebathing prior to heating these was a heat distribution ofintervals.




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Fabrication Related Theory for Exercise 2.3.88Welder - Gas tungstan arc welding

Principles of Gas Tungsten Arc Welding (GTAW), advantages & limitationsObjectives: At the end of this lesson you shall be able to• state the principle of GTAW process• state its application• state the advantages and disadvantages of GTAW.

Process description (Fig.1)Gas Tungsten Arc Welding (GTAW), also known as tung-sten inert gas (TIG) welding is a process that produces anelectric arc maintained between a nonconsumable tung-sten electrode and the part to be welded. The heat-af-fected zone, the molten metal, and the tungsten elec-trode are all shielded from atmospheric contamination bya blanket of inert gas fed through the GTAW torch. Inertgas (usually Argon) is inactive or deficient in active chemi-cal properties. The shielding gas serves to blanket theweld and exclude the active properties in the surroundingair. Inert gases, such as Argon and Helium, do not chemi-cally react or combine with other gases. They pose noodor and are transparent, permitting the the welder maxi-mum visibility of the arc. In some instances Hydrogen gasmay be added to enhance travel speeds.

The GTAW process can produce temperatures of up to3000° F. The torch contributes heat only to the workpiece.If filler metal is required to make the weld, it may be addedmanually in the same manner as it is added in the oxy-acetylene welding process, or in other situations may beadded using a cold wire feeder.

GTAW is used to weld steel, stainless steel, nickelalloys, titanium, aluminum, magnesium, copper, brass,bronze, and even gold. GTAW can also weld dissimilarmetals to one another such as copper to brass and stain-less steel to mild steel.

Advantages of GTA welding

• Concentrated Arc - Permits pinpoint control of heat in-put to the workpiece resulting in a narrow heat-affectedzone.

• No Slag - No requirement for flux with this process;therefore no slag to obscure the welder’s vision of themolten weld pool.

• No Sparks or Spatter - No transfer of metal across thearc. No molten globules of spatter to contend with andno sparks produced if material being welded is free ofcontaminants.

• Little Smoke or Fumes - Compared to other arc-weld-ing processes like stick or flux cored welding, few fumesare produced. However, the base metals being weldedmay contain coatings or elements such as lead, zinc,copper, and nickel that may produce hazardous fumes.Keep your head and helmet out of any fumes rising offthe workpiece. Be sure that proper ventilation is sup-plied, especially in a confined space.

• Welds more metals and metal alloys than any otherarc welding process.

• Good for welding thin material.

• Good for welding dissimilar metals together.

Disadvantages of GTA welding

• Slower travel speeds than other processes.

• Lower filler metal deposition rates.

• Hand-eye coordination is a required skill.

• Brighter UV rays than other processes.

• Equipment costs can be higher than with other pro-cesses.

• Concentrations of shielding gas may build up and dis-place oxygen when welding in confined areas - venti-late the area and/or use local forced ventilation at thearc to remove welding fumes and gases. If ventilationis poor, wear an approved air-supplied respirator.


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Fabrication Related Theory for Exercise 2.3.89Welder - Gas tungsten arc welding

GTAW process brief descriptionObjective: At the end of this lesson you shall be able to• difference between equipments polarities and applications.

Working principle: The output of the step down trans-former is connected to the rectifier unit, which converts ACto DC. The DC output is connected to positive and negativeterminals, from where it is taken for welding purposesthrough welding cables. It can be designed to provideeither AC or DC welding supply by operating a switchprovided on the machine.

Care and maintenance of rectifier welding setKeep all the connections in tight condition.

Lubricate the fan shaft once in 3 months.

Do not adjust the current or operate the AC/DC switchwhen the welding arc is ‘on’.

Keep the rectifier plates clean.

Check and clean the set atleast once in a month.Keep the air ventilation system in good order.

Never run the machine without the fan.

Constructional features of AC/DC welding rectifier: Awelding rectifier set is used to convert AC welding supplyinto DC welding supply. It consists of a step down trans-former and a welding current rectifier cell with a cooling fan.(Fig.1) The rectifier cell consists of a supporting plate madeof steel or aluminium (Fig.2) which is plated with a thin layerof nickel or bismith, sprayed with SELENIUM or SILICON.It is finally covered with an alloyed film of CADMIUM,BISMITH and TIN.

The coating of nickel or bismuth over the supporting plateserves as one electrode (ANODE) of the rectifying cell. Thealloyed film (of cadmium, bismuth and tin) serves asanother electrode (CATHODE) of the rectifying cell. Therectifier acts as a non-return valve and allows current to flowon one side of it as it offers very little resistance and on theother side it offers very high resistance to the flow of thecurrent. Hence the current can flow in one direction only.

Advantages and disadvantages of AC and DC weldingObjectives: At the end of this lesson you shall be able to• compare the advantages and disadvantages of AC welding• compare the advantages and disadvantages of DC welding.Advantages of AC weldingA welding transformer has– a low initial cost due to simple and easy construction– a low operating cost due to less power consumption– no effect of arc blow during welding due to AC– low maintenance cost due to the absence of rotating

parts– higher working efficiency– noiseless operation.

Disadvantages of AC weldingIt is not suitable for bare and light coated electrodes.

It has more possibility for electrical shock because ofhigher open circuit voltage.

Welding of thin gauge sheets, cast iron and non-ferrousmetals (in certain cases) will be difficult.

It can only be used where electrical mains supply isavailable.


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Advantages of DC weldingRequired heat distribution is possible between the elec-trode and the base metal due to the change of polarity(positive 2/3 and negative 1/3).

It can be used successfully to weld both ferrous and non-ferrous metals.

Bare wires and light coated electrodes can be easily used.

Positional welding is easy due to polarity advantage.

It can be run with the help of diesel or petrol engine whereelectrical mains supply is not available.

It can be used for welding thin sheet metal, cast iron andnon-ferrous metals successfully due to polarity advantage.

It has less possibility for electrical shock because of lessopen circuit voltage.

It is easy to strike and maintain a stable arc.

Remote control of current adjustment is possible.

Disadvantages of DC weldingDC welding power source has:– a higher initial cost– a higher operating cost– a higher maintenance cost– trouble of arc blow during welding– a lower working efficiency– noisy operation in the case of a welding generator– occupies more space.

GTAW process and equipmentObjectives: At the end of this lesson you shall be able to• identify the GTAW equipment• name the parts of a GTAW equipment

TIG welding equipment (Fig 1)– An AC or DC arc welding machine.– Shielding gas cylinders or facilities to handle liquid

gases– A shielding gas regulator– A gas flowmeter– Shielding gas hoses and fittings

– A welding torch (electrode holder)– Tungsten electrodes– Welding filler rods– Optional accessories– A water cooling system with hoses for heavy duty

welding operations– Foot rheostat (switch)



64

GTAW power sourcesObjectives: At the end of this lesson you shall be able to• state the necessity of a suitable power sources• state the different types of power sources used• state the application of different power sources• state the care and maintainance of GTAW machine.

Power sourcesTIG welding power sources have come a long way fromthe basic transformer types of power sources which wereused with add-on units to enable the power source to beused as a TIG unit, eg high frequency unit and/or DC rec-tifying units.The basics of TIG welding has almost remained the same,but the advent of technology TIG welding power sourceshave made the TIG processes more controllable and moreportable.The one thing that all TIGs have in common is that theyare CC (Constant Current) type power sources. This meansonly output adjustment will control the power source amps.The voltage will be up or down depending on the resis-tance of the welding arc.

History of Gas Tungsten Arc Welding (GTAW)GTAW welding was, like GMAW developed during 1940at the start of the Second World War.

GMAW’s development came about to help in the weldingof difficult types of material, eg aluminium and magne-sium. The use of GMAW today has spread to a variety ofmetals like stainless mild and high tensile steels.

GTAW is most commonly called TIG (Tungsten Inert Gaswelding).

The development of TIG welding has added a lot in theability to make products, that before the 1940’s were onlythought of.

Like other forms of welding, TIG power sources have, overthe years, gone from basic transformer types to the highlyelectronic power source of the world today.

OverviewTIG welding is a welding process that uses a power source,a shielding gas and a TIG torches. The power is fed out ofthe power source, down the TIG torches and is deliveredto a tungsten electrode which is fitted into the torches. Anelectric arc is then created between the tungsten elec-trode and the workpiece. The tungsten and the weldingzone is protected from the surrounding air by a gas shield(inert gas). The electric arc can produce temperatures ofup to 30000C and this heat can be very focused local heat.

Various other name of the process (Tig orgamic)Objectives: At the end of this lesson you shall be able to• understand the history and overview of GTAW• state the advantages and applications of GTAW

The weldpool can be used to join the base metal with orwithout filler material.

The TIG process has the advantages of -1 Narrow concentrated arc2 Able to weld ferrous and non-ferrous metals3 Does not use flux or leave a slag4 Uses a shielding gas to protect the weldpool and tung-

sten5 A TIG weld should no spatter6 TIG produces no fumes but can produce ozone

The TIG process is a highly controllable process that leavesa clean weld which usually needs little or no finishing. TIGwelding can be used for both manual and automatic op-erations.

The TIG welding process is so good that it is wisely usedin the so-called high-tech industry applications such as

1 Nuclear industry2 Aircraft3 Food industry4 Maintenance and repair work5 Some manufacturing areas6 The off shore industry7 Combined heat and power plants8 Petro chemical industry.9 Chemical industry.


Characteristerics of power fource :The output slope orvoltampere curve A, a change from 20 volts to 25 volts willresult in a decrease in amperage from 135 amps to 126amps. With a change of 25 percent in voltage, only a 6.7percent change occurs in the welding current in curve A.Thus if the welder varies the length of the arc, causing achange in voltage, there will be very little change in thecurrent and the weld quality will be maintained. The currentin this machine, even though it varies slightly is consideredconstant (Fig 1).

This is called drooping characteristic power source. Alsocalled Constant Current (CC)power source.

This type of power source is used in SMAW & GTAWprocess.


65

Types of welding current used for GTAWWhen TIG welding, there are three choices of welding cur-rent. They are: Direct Current Straight Polarity, Direct Cur-rent Reverse Polarity, and Alternating Current with HighFrequency stabilisation. Each of these has its applica-tions, advantages, and disadvantages. A look at each typeand its uses will help the operator select the best currenttype for the job. The type of current used will have a greateffect on the penetration pattern as well as the bead con-figuration. The diagrams below, show arc characteristicsof each current polarity type.

DCSP - Direct Current Straight Polarity (Fig 2): (Thetungsten electrode is connected to the negative terminal).This type of connection is the most widely used in the DCtype welding current connections. With the tungsten be-ing connected to the negative terminal it will only receive30% of the welding energy (heat). This means the tung-sten will run a lot cooler than DCRP. The resulting weldwill have good penetration and a narrow profile.

Cueent type DCRPElectrode Polarity Electrode Positive

Oxide Cleaning Action Yes

Heat Balance in the Arc 30% at work end 70% at electrode end

Penetration Profile Shallow, wide

Electrode Capacity Poor


Current type DCSPElectrode Polarity Electrode NegativeOxide Cleaning Action NoHeat Balance in the Arc 70% at work end 30% at

electrode endPenetration Profile Deep, narrowElectrode Capacity Excellent

DCRP - Direct Current Reverse Polarity (Fig 3): (thetungsten electrode is connected to the positive terminal).This type of connection is used very rarely because mostheat is on the tungsten, thus the tungsten can easily oveheat and burn away. DCRP produces a shallow, wide pro-file and is mainly used on very light material at low amps.

Current type ACHFElectrode Polarity Alternating

Oxide Cleaning Action Yes (once every half cycle)

Heat Balance in the Arc 50% at work end 50% at electrode end

Penetration Profile Medium

Electrode Capacity Good

AC - Alternating Current (Fig 4) is the preferred weldingcurrent for most white metals, eg aluminium and magne-sium. The heat input to the tungsten is averaged out asthe AC wave passes from one side of the wave to theother.

On the half cycle, where the tungsten is positive electronwelding current will flow from base material to the tung-sten. This will result in the lifting of any oxide skin on thebase material. This side of the wave form is called thecleaning half. As the wave moves to the point where thetungsten becomes negative the electrons (welding cur-rent) will flow from the welding tungsten to the base mate-rial. This side of the cycle is called the penetration half ofthe AC wave form.


66

Because the AC cycle passes through a zero point thearc goes out. This can be seen with fast film photography.At this point the arc would stay out if it wasn’t for theintroduction of HF (high frequency). High frequency hasvery little to do with the welding process; its job is the re-ignition of the welding current as it passes through zero.HF is also often used for starting the welding arc initiallywithout the tungsten touching the workpiece. This helpson materials that are sensitive to impurities. HF start canalso be used on DC welding current to initially start thewelding current without the tungsten touching theworkpiece.

AC - Alternating Current - Square Wave (Fig 5)


ACPowerSupply

With the advent of modern electricity AC welding machinescan now be produced with a wave form called Square Wave.The square wave has the benefit of a lot more control andeach side of the wave can be, in some cases, controlledto give a more cleaning half of the welding cycle, or morepenetration.

Once the welding current gets above a certain amperage(often depends on the machine) the HF can be turned off,allowing the welding to be carried on with the HF interfer-ing with anything in the surrounding area.

Extended Balance Control (Fig 5 ,6 & 7)AC balance control allows the operator to adjust the bal-ance between the penetration (EN) and cleaning action

(EP) portions of the cycle. Some inverters have adjust-able EN as great as 30 percent to 99 percent for controland fine-tuning of the cleaning action.

For instance, if the operator sets EN at 60 percent, itmeans that 70 percent of the AC cycle is putting energyinto the work, while 40 percent of the cycle is cleaning.

Pulsed TIG (Fig 8)In this type of power source, the supply current is notconstant and it is being fluctuated from low level to highlevel. This causes low heat input to the metal and hencedistortion effect will be less.

Pulsed TIG has the advantages of1 better penetration with less heat2 less distortion3 better control when welding out of position4 easy to use on thin materials



The down side is - more set-up cost and more operatortraining.

Pulsed TIG consists of

Peak current - This is set up higher than for non-pulsedTIG.

Background current - This is set lower than peak cur-rent and is the bottom current the pulse will drop to, butmust be enough to keep the arc alive.

Pulses per second - This is the number of times persecond that weld current reaches peak current.

% on Time - This is the pulse peak duration as a percent-age of the total time, which controls how long the peakcurrent is on for before dropping to the background cur-rent.

The pulse and base current periods are also controllable.

When welding is done with pulsing welding mode the weldis in principle a row of spot welds overlapping to a lerger orsmaller extent depending on the welding speed.

Many double-current machines are equipped with a con-trol function which makes it possible to modify the curveof the alternating current in balance between thew positiveand the negative semi-periods.

Current Type DCEN DCEP AC (Balanced)

Electrode Polarity Negative Positive

Electron and

ion flow

Penetration

Charateristics

Oxide Cleaning Yes-once every

Action No Yes Half Cycle

Heat Balance in 70% at work end 30 % at work end 50 % at work end

the arc (approx.) 30% at electrode end 70% at electrode end 50 % at electrode end

Penetration Deep Narrow Shallow Wide Medium

Electrode Excellent Poor Good

Capacity e.g., 1/8 in. (3.2 mm) 400 A e.g. 1/4 in. (6.4 mm) 120 A e.g. 1/8 in. (3.2 mm) 225 A


68

Electrodes for TIG weldingFor TIG welding the applied electrode is mainly made oftungsten.

Pure tungsten is a very heat resistance material with afusion point of approximately 3,3800C.

By alloying tungsten with a few per cent of a metal oxidethe conductivity of the electrode can be increased whichhas the advantage that it can thereby resist a higher cur-rent load.

The alloyed tungsten electrodes therefore have a longerlifetime and better ignition properties than electrodes ofpure tungsten.

The most frequently used metal oxides used for alloyingof tungsten are:

• Thorium oxide ThO2

• Zirconium oxide ZrO2

• Lanthanum oxide LaO2

• Cerium oxide CeO2

Colour indications on tungsten electrodesAs the pure tungsten electrodes and the different alloyedones look the same, it is impossible to tell the differencebetween them. Therefore a standard colour indication onthe electrodes has been agreed.

The electrodes are marked with a particular colour on thelast 10 mm.

The most commonly used types of tungsten electrodesare:

• Pure tungsten is marked with green colour. This elec-trode is especially used for AC welding in aluminiumand aluminium alloys.

• Tungsten with 2% thorium is marked with red colour.This electrode is mostly used for welding of non-alloyedand low-alloyed steels as well as stainless steels.

• Tungsten with 1% lanthanum is marked with blackcolour. This electrode is equally suited for welding of allTIG weldable metals.

Colour code and alloying elements for various tungsten electrode alloys

AWS classifications Colour* Alloying element Alloying oxide Current type

EWP Green Pure - AC/DC

EWCe-2 Orange Cerium CeO2 AC/DC

EWLa-1 Black Lanthanum La2O3 AC/DC

EWTh-1 Yellow Thorium ThO2 DC

EWTh-2 Red Thorium ThO2 DC

EWZr-1 Brown Zirconium ZrO2 AC

• Colour may be applied in the form of bands, dots, etc,at any point on the surface of the electrode.

Electrode dimensionsTungsten electrodes are available in different diametersfrom 0.5 to 8 mm. The most frequently used dimensionsfor TIG welding electrodes are 1.6 - 2.4 - 3.2 and 4 mm.

The diameter of the electrode is chosen on basis of thecurrent intensity, which type of electrode that is preferredand whether it is alternating or direct current.

Grinding angleAn important condition for obtaining a good result of TIGwelding is that the point of the tungsten electrode must beground correctly.

When welding is done with direct current and negativepolarity, the electrode point should be conical in order toobtain a concentrated arc that will provide a narrow anddeep penetration profile.

The following thumb rule indicates the relation betweenthe diameter of the tungsten electrode and the length ofits ground point.


GTAW electrodesObjectives: At the end of this lesson you shall be able to• state the types of electrodes• state the colour codification.


69

A small pointed angle gives a narrow weld pool and thelarger the pointed angle the wider the weld pool (Fig 1).

The pointed angle also has an influence of the penetrationdepth of the weld (Fig 2).

Blunting the electrode point to make a flat area with adiameter of about 0.5 mm can increase the lifetime of thetungsten electrode (Fig 3).

For AC TIG welding the tungsten electrode is rounded asduring the welding process it is so heavily loaded that it ismelted into a half globular form (Fig 4).

Grinding of the tungsten electrodeWhen grinding the electrode its point must point in thedirection of the rotation of the grinding disc so the grindingtraces will lie lengthways the electrode (Fig 5, 6, 7).



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Electrode condition: Fig. 8 shows tungsten electrodeconditions associated with TIG welding.

Commentsa Well sharpened and healthy electrode (color ‘silver

white’) and used with normal current. Sharpening to acone (without a point) allows a rapidly forming andstable arc, centered in relation to the electrode.

b The point of the electrode has melted under the actionof too great a current. The point is deformed, the arc iserratic and poorly directed because the ball ‘vibrates’during welding. Welding is therefore difficult, if notimpossible.

c The electrode has been used without protection ofargon shielding gas. The flow has been cut off too soon.The electrode has turned blue, is contaminated withoxygen and disintegrates rapidly. It is necessary toreshape it.

d This fault occurs mostly in the welding of light alloyswith an electrode of thoriated tungsten and a lowcurrent. The current must be increased to form a ballshape at the electrode tip. If this is not done the arc willremain ‘erratic.

e Electrode point too sharp. Rapid wear occurs since thepoint carries current densities which are too high. thisleads to systematic inclusions of tungsten in the weldwhich are highly visible on radiographics.

Tungsten selection and preparation

Base metal type

Aluminium alloys andMagnesium alloys

Copper Alloys, Cu-Ni alloysand nickel alloys

Mild steels, Carbon steels,alloy steels and Titanium alloys

Welding current Electrode type Shield gas

2% Thoriated (EW-Th2 Argon

2% Ceriated(EW-Ce2 Argon, Helium mixture

DCSP

2% Thoriated (EW-Th2) Argon

2% Ceriated(EW-Ce2 Argon, Helium mixture

2% Lanthanated (EWG-Th2)

AC/HF

DCSP

Pure (EW-P) Argon

Zirconiated(EW-Zr) Argon

Argon

GTAW electrodes: The electrode is made of tungsten oran alloy of it and this provides easy arc starting and steadyarcing. The Fig.9 show the recommended tip shape for DCEN polarity and AC polarity for TIG welding.

The common varieties of the tungsten electrodes are

– pure tungsten

– 1% or 2% thorium oxide and tungsten

Tungsten with thorium oxide is used welding with DC. Anaddition of 1% or 2% thorium oxide increases the maxi-mum current carrying capacity by approx. 45-50% for agiven electrode and does not form hemispherical blobs asdoes the pure tungsten.

Thoriated tungsten electrodes are however preferred for DCas the arc wanders when used on AC. Red and yellowcolour bands are used widely to indicate 2% and 1%alloyed thoriated tungsten electrodes. The current carryingcapacity depends upon the type of shielding gas (whetherargon or helium).



71

GTAW torchTorch: There is a variety of torches available varying fromlight weight air cooled to heavy duty water cooled types.Figs1 & 2. The main factors to be considered in choosinga torch are:

– Current carrying capacity for the work in hand

– Weight, balance and accessibility of the torch head tothe work in hand.

The torch body holds a top loading compression-type colletassembly which accommodates electrodes of variousdiameters. They are securely gripped, yet the collet iseasily slackened for removal or reposition of the electrode.As the thickness of plate to be welded increases, size oftorch and electrode diameter must increase to deal with thelarger welding currents required.


GTAW torches - types, parts and their functionsObjectives: At the end of this lesson you shall be able to• state the purpose of the torch and its parts• state the care and maintenance of torches.

Parts of water cooled torch Fig.21 Thoriated or Zirconiated tungsten electrode 2 Ceramic shield/nozzle 3. "O" ring 4 Collet holder5 Collet 6 Electrode cap (short & long) 7 Body assembly8 Sheath 9 Hose assembly cover 10 Argon hose assembly11 Water hose assembly 12 Power cable assembly 13 Adaptor (power cable)14 Adaptor (argon gas hose) 15 Switch actuator 16 Switch17 Switch retaining sheath 18 Cable (2 core) 19 Insulating sleeve20 Plug


72

The function of the TIG torch is to

1 hold the electrode tungsten

2 deliver welding current to the tungsten via a weldingpower cable

3 deliver shielding gas to the TIG torch nozzle. The nozzlethen directs the shielding gas to cover the weldpoolprotecting it from contamination from the surroundingair.

4 often will be the way of getting the welder control cir-cuit to the operation, eg on/off and/or amperage con-trol.

5 the TIG torch can be watercooled. Hoses in the TIGlead will supply cooling water to the TIG torch headassembly.

6 the TIG torch length will allow a distance from the TIGpower source and workpiece.

TIG torches come in different styles depending on the brandbeing selected. But they all have things in common -

1 aircooled or watercooled

2 current rating. The operator must select the correctamperage rating TIG torch.

Cooling of the TIG torchSome torches are constructed in such a way that it is theflowing shielding gas that cools the torch. However, thetorch also gives off heat to the surrounding air.

Other torches are constructed with cooling tubes. Water-cooled torches are mainly used for welding with largercurrent intensities and AC-welding.

Usually a water-cooled TIG torch is smaller than an air-cooled torch designed to the same maximum current in-tensities

Using a TIG torch that is not sufficiently rated for the ma-chine may result in the TIG torch overheating. A TIG torchwiith an excessive rating may be larger and heavier than alower amperage TIG torch.

The TIG torch is made up of

1 Leads - The lead will be set up for either aircooled orwatercooled. It will be at a length suitable to do thejob, eg 4 metre, 8 metre, etc. The lead will be made upof a power cable, gas hose and water leads in and outif the TIG torch is watercooled. The lead may also in-clude a control lead.

2 Collet - To hold the tungstan rods. Collet may varywith different brands of TIG torches.

3 Ceramic Nozzles - The nozzle’s job is to direct thecorrect gas flow over the weldpool.

4 Back Caps - The back cap is the storage area for ex-cess tungsten. They can come in different lengths de-pending on the space the torch may have to get into(eg. long, medium and short caps).

Please make sure when ordering a TIG torch to tell thesupplier the amperage rating, whether water- or air-cooled,and the fitting that is to go on the end of the TIG torch leadsuitable to fit the TIG power source it will be used from.This may include power cable fit up, gas fittings and con-trol plug fittings.

Gas regulator & flowmeterGas regulator, flowmeter (Fig 3 & 4): The gas regulatorreduces the pressure in the argon cylinder from 175 or 200bar down to 0-3.5 bar for supply to the torch.

The flowmeter which has a manually operated needle valve,controls the argon flow from 0-600 litres/hour to 0-2100litres/hour according to type.



73

GTAW filler rods and selection methods (criteria)Objective: At the end of this lesson you shall be able to• state the mandatory classification designators.

In the welding process (GTAW or gas tungsten) is an arcwelding process taht operates the filler rods.

The TIG torch may be cooled by air or water and the pro-cess uses a filler metal in road form. The tungsten elec-trode selection and parameters for welds are guided them.

Welding filler metal designators1 Carbon steel electrodes

Mandatory classification designatorsDesignates an electrode

Designates minimum tensile strength, in Ksi, of the as-deposited weld metal.

Designates the welding position, the type of covering andthe type of welding current for which the electrodes aresuitable (See table below)

Optional supplemental designatorsDesignates that the electrode meets the requirements ofabsorbed moisture.

Designates that the electrode meets the requirements ofthe diffusible hydrogen test - with an average value noexceeding "Z" mL of H2 per 100gms of deposited metal.

Designates that the electrode meet the requirements forimproved toughness and ductility.

E XX YY - 1 HZ R

Optional supplemental designators

AWS Type of covering Welding position Type of current b Classification

E6010 High cellulose, sodium F,V,OH, H dcepE 6011 High cellulose, potassium F,V,OH,H as or dcepE 7018 Low hydrogen, Potassium, F,V,OH,H ac or dcep

PowderE7024 Iron Powder, Titania H-Fillets, F ac, dcep or dcen

Gas tungsten arc welding also know as tungsten inertgas (TIG) welding, is an arc development within theGTAW process.

Now always the filler rods is withdraw from the weld pooleach time the electrode can be changed.

Note

a The abbreviations indicate the welding positions

F=Flat; V=Vertical, OH=overhead, H=Horizontal,H=Fillets = Horizontal fillets.

b The term dcep refers to direct current electrode positive(dc, straight polarity)

Also note that the above electrode classifications are themost widely used and does not include all of the availableclassifications. Refer to AWS A 5.1 for complete listing.



74

3 Stainless steel filler metalUsability classification

2 Alloy steel electrodesMandatory classification designators

Types of welding current and position of weldingAWS classification Welding current Welding position

EXXX (X) - 15 dcep All

EXXX (X) - 16 dcep or ac All

EXXX (X) - 17 dcep or ac All

EXXX(X) - 25 dcep H,F

EXXX (X) - 26 dcep or ac H,F

For more details on the usability classifications, refer toAWS A 5.4

Designates and electrode

Designates minimum tensile strength, in Ksi, of the as-deposited weld metal

Designates the welding position, the type of covering andthe type of welding current for which the electrodes aresuitable.

Designates the chemical composition of the undiluted weldmetal produced by the electrode using SMAW process.

Optional supplemental designatorsDesignates that the electrode meets the requirements ofabsorbed moisture.

Designates that the electrode meets the requirements ofthe diffusible hydrogen test - with an average value notexceeding "Z" mL of H2 per 100gms of deposited metal,where "Z" is 4,8 or 16.

Refer to AWS A 5.5 for complete listing of mechanicalproperties, chemical composition of as deposited weldmetal and testing procedures for SMAW process.

E (X) XX YY- X HZ R

Table 1: Carbon and low - alloy steel welding consumablesfor SMAW process

Types of welding current and position of welding

Car

bon

stee

l

Car

bon-

mol

ybde

num

stee

l 1 an

d 1

1/4

Cr-

1/2

Mo

stee

l

2 1/

4 C

r-1

Mo

stee

l

5 C

r-1/

2 M

o St

eel

9 C

r - 1

Mo

stee

l

Base material

Carbon steel AB AC AD AE AF AG

Carbon-Molybdenum steel C CD CE CF CH

1 and 1 1/4 Cr-1/2 Mo steel D DE DF DH

2 1/4 Cr-1 Mo steel E EF EH

5 Cr - 1/2 Mo steel F FH

9 Cr-1 Mo steel H



75

LegendA AWS A 5.1 classification E 70XX low hydrogen

(E7018 preferred)

B AWS A 5.1 classification E 70XX low hydrogen(E7018 preferred)

C AWS A 5.5 classification E70XX - A1, low hydrogen

D AWS A 5.5 classification E70XX - B2L or E80XX-B2, low hydrogen

E AWS A 5.5 classification E80XX-B3L or E80XX-B6L, low hydrogen

F AWS A 5.5 classification E80XX-B6 or E80XX-B6L,low hydrogen

G AWS A 5.5 classification E80XX-B7 or E80XX-B7L,low hydrogen

H AWS A 5.5 classification E90XX-B8 or E80XX-B8L,low hydrogen

1 Table 1 refers to coated electrodes (SMAW process)only. For bare wire welding (SAW, GMAW, GTAW andFCAW), use equivalent electrode classifications (AWSA 5.14, A 5.17, A5.18, A 5.20, A 5.23, At 28)

2 Higher allow electrode specified in the table shouldnormally be used to meet the required tensile andtoughness after post weld heat treatment (PWHT). Ifno PWHT is required, the lower alloy electrode specifiedmay be required to meet the hardness requirements.

Table 2: Austenitic, super-austenitic and duplex stainless steel alloys

Carbon and low alloy steel ABC ABC ABC ABC ABC ABC ABC ABC N N

Type 304L stainless stell D DE DF DG DC C C DCH NL NL

Type 304H stainless steel E EF EG * * * ECH * *

Type 316L stainless steel FG FG FC FC FC FCH NL NL

Type 317L stainless steel GC GC GC GC GC L L

Type 904L stainless steel C C C C L L

Type 6% Mo stainless steel CJK CJK * * *

Eg: 254 SMO, AL 6XN CJK * * *

Type Alloy 20Cb-3 H * *

Type 2304 Duplex SS LM LM

Type 2205 Duplex SS LM

Types of welding current and position of welding

304L

SS

317L

SS

904L

SS

6% M

o SS

7% M

o SS

Allo

y 20

Cb-3

Base Material30

4H S

S

316L

SS

2304

Dup

lex

SS

2205

Dup

lex

SS

LegendA-AWS A 5.4 classification E309L-XXB-AWS A 5.11 classification ENiCrFe-2 or -3(-2 is alloy 718 and -3 is inconel 182)C-AWS A 5.11 classification ENiCrMo-3 (Inconel 625)D-AWS A 5.4 classification E308L-XXE-AWS A 5.4 classification E308H-XXF-AWS A 5.4 classification E316L-XXG-AWS A 5.4 Classification E317L-XXH-AWS 5.4 classification E320LR-XXJ-AWS A5.11 classification ENiCrMo-4(Hastelloy C-276)

K-AWS A 5.11 classification ENiCrMo-11(Hastelloy G-30)L-AWS A 5.4 classification E2209-XXM-AWS A 5.4 classification E2553-XXN-AWS A 5.4 classification E309MoL-XX

Table 2 refers to coated electrodes only. Forwire welding (GMAW & GTAW) use equivalentelectrode classification (AWS A5.14)There are many proprietary alloys availablein the market and material combinations youmight encounter. Consult the manufacturer orthe DFD for proper filler metal selection.



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GTAW/TIG welding is generally recommended for pipe topipe or tube to tube joints. TIG welding with inert gasshielding produces the joints without any defect like gasporosity, oxide slag inclusions and hence the joints are ofsuperior quality.

For MS/Carbon steel pipes and tubes, when welding isdone with TIG process, the weld metal is free fromhydrogen entrapment which usually occurs in normal oxyacetylene gas welding or manual metal arc welding proc-esses. The hydrogen gas dissolved in the weld causesembrittlement during service. Hence TIG welding for MSpipes is always preferred for all pipe lines viz., gas pipelines/liquid lines in all petroleum and power plant to conveyhigh temperature and high pressure fluids (liquids &gases,steam etc.).

There are various pipe jointings like straight butt welds, fillettee joints and pipe elbow joints to suit the piping layout ofany process plant say petroleum or power generatingplant.

Therefore it is mandatory to take utmost care in develop-ment of members of pipe joints so that the geometry willprovide appropriate clearances for the joint fit up and theTIG welds so produced will be free from any defect and willoffer highest joint efficiency as per the design standards.

Various configuration of the joints are well shown inpractical Exercise book.

Fabrication Related Theory for Exercise 2.3.92Welder - Gas tungsten arc welding

Edge preparations and GTAW parameters for different type of metalsObjective: At the end of this lesson you shall be able to• state the welding method of MS pipe using TIG welding process.

Low alloy steel (DCSP)

Metal Joint Tungsten Filler Rod Cup Shield Gas Flow Welding TravelGauge Type size Size Size Type CFH PSI Amperes Speed

(L/Min)

1.6 mm Butt 1.6 mm 1.6 mm 4, 5, 6 Argon 15 (7) 20 95-135 400 mmFillet

3.2 mm Butt 1.6 mm 2.4 mm 4, 5, 6 Argon 15 (7) 20 145-205 300 mmFillet 2.4 mm

4.8 mm Butt 2.4 mm 3.2 mm 7, 8 Argon 16 (6.5) 20 210-260 250 mmFillet

6.4 mm Butt 3.2 mm 4.0 mm 8, 10 Argon 18 (8.5) 20 240-300 250 mmFillet(2)

Welding low alloy steelMild and low carbon steels with less than 0.30% carbonand less than 25 mm thick, generally do not require pre-heat. Low alloy steels such as the chromium-molybde-num steels will have hard heat affected zones after weld-ing, if the preheat temperature is too low. This is causedby rapid cooling of the base material and the formation ofmartensitic grain structures. A 900 to 2000C preheat tem-perature will slow the cooling rate and prevent the marten-sitic structure.


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Typical manual GTA (TIG) welding parameters

Aluminium (ACHF)

Metal Joint Tungsten Filler Rod Cup Shield Gas Flow Welding TravelThickness Type size Size Size Type CFH PSI Amperes Speed

(L/Min) mm/min

1.6 mm Butt 1.6 mm 1.6 mm 4, 5, 6 Argon 15 (7) 20 60-80 300Fillet 70-90 250

3.2 mm Butt 2.4 mm 2.4 mm 6, 8 Argon 17 (8) 20 125-145 300Fillet 3.2 mm 140-160 250

4.8 mm Butt 3.2 mm 3.2 mm 6, 8 Argon/ 21 (10) 20 190-220 250Fillet Helium 210-240 230

6.4 mm Butt 4.8 mm 3.2 mm 8, 10 Argon/ 25 (12) 20 260-300 250Fillet Helium 280-320 200

Welding aluminiumThe use of TIG welding for aluminium has many advan-tages for both manual and automatic processes. Fillermetal can be either wire or rod and should be compatiblewith the base alloy.

CFH - Cubic Feet per hour ACHF - Alternative current with high frequency L/Min - Litre per minute


Magnesium (ACHF)

Metal Joint Tungsten Filler Rod Cup Shield Gas Flow Welding TravelGauge Type size Size Size Type CFH PSI Amperes Speed

(L/Min) mm/min

1.6 mm Butt 1.6 mm 2.4 mm 5, 6 Argon 13 (5) 15 60 500 Fillet 3.2 mm 60

3.2 mm Butt 2.4 mm 3.2 mm 7, 8 Argon 19 (9) 15 115 450 Fillet 4.0 mm 115

6.4 mm Butt 4.8mm 4.0 mm 8 Argon 25 (12) 15 100-130 550 Butt(2) 110-135 500

12.8 mm Butt(2) 6.4 mm 4.8 mm 10 Argon 35 (17) 15 260 250

Welding magnesiumMagnesium alloys are in three groups, they are (1) alu-minium-zinc-magnesium, (2) aluminium-magnesium, and(3) maganese-magnesium. Since magnesium absorbs anumber of harmful ingredients and oxidize rapidly whensubjected to welding heat, TIG welding in an inert gasatmosphere is distinctly advantageous, the welding ofmagnesium is similar, in many respects, to the welding ofaluminium. Magnesium was one of the first metals to bewelded commercially by TIG. Magnesium requires a posi-tive pressure of argon as a backup on the root side of theweld.

Filler metal must be dry, free of oxides, grease, or otherforeign matter.

Types of welding jointsThere are five basic joint designs mentioned below.

1 Butt joint2 Lap joint3 corner joint4 T joint (Fillet joint)5 Edge joint

The Fig. 1 below shows, filler T joint welded in flat position,When the weld axis and weld face are horizontal.


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Fig.4 shows the setup for fillet tee joint on stainless steelsheets, welded by (TIG) GTAW process. Satisfactoryweld, should be free from undercut at the toes and of equalleg lengths.

Fig. 6a,b& c elaborates the recommended steps by GTAWprocess using filler rod.

In this method, the two pieces of metal do not overlap.While welding the outside corner joint, watch the outsideedges of the metal. When the puddle touches the outsideedges of the metal, the welding rod should be placed in thepuddle. Enough filler metal should be melted into thepuddle to form a convexed bead. The torch is moved forwardand the welding rod is again placed in the puddle when thepuddle touches the outside edges of the joint. This opera-tion is repeated until the joint is completely welded. Thekeyhole welding technique may be used on this joint. Fig.7shows a macrograph of a completed outside corner jointmade with proper and improper assembly of a corner joint.

Fig. 5 shows the corner joint welded by TIG process. It alsogives the set up for making the outside corner joint in flateposition.

Square butt joint on aluminium: The Fig. 2 shows therecommended torch and welding filler rod positions forwelding a butt joint in flat position.

Outside corner joint on aluminium: Fig.3 on the nextpage shows a typical corner joint welded from outside.



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Welding stainless steelIn TIG welding of stainless steel, welding rods having theAWS-ASTM prefixes of E or ER can be used as filler rods.However, only bare uncoated rods should be used. Stain-less steel can be welded using ACHF, however, recom-mendations for DCSP must be increased 25%. Light gaugemetals less than 1.6 mm thick should always be weldedwith DCSP using argon gas. Follow the normal precau-tions for welding stainless such as: Clean surfaces; dryelectrodes; use only stainless steel tools and brushes,carefully remove soap from welds after pressure testing;keep stainless from coming in contact with other metals.

Stainless steel (DCSP) Welding parameters

Metal Joint Tungsten Filler Rod Cup Shield Gas Flow Welding Travel Gauge Type size Size Size Type CFH PSI Amperes Speed

(L/Min) mm/min

1.6 mm Butt 1.6 mm 1.6 mm 4, 5, 6 Argon 11 (5.5) 20 80-100 300Fillet 90-100 250

3.2 mm Butt 1.6 mm 2.4 mm 4, 5, 6 Argon 11 (5.5) 20 120-140 300Fillet 130-150 250

4.8 mm Butt 2.4 mm 3.2 mm 5, 6, 7 Argon 13 (6) 20 200-250 300Fillet 2.4 mm 225-275 250

3.2 mm

6.4 mm Butt 3.2 mm 4.8 mm 8, 10 Argon 13 (6) 20 275-350 250Fillet 300-375 200

Pulsed TIG welding and description of pulse param-etersIn this type of power source, the supply current is notconstant and it is being fluctuated from low level to highlevel. This causes low heat input to the metal and hencedistortion effect will be less.

Pulsed TIG has the advantages of

1 better penetration with less heat

2 less distortion

3 better control when welding out of position

4 Easy to use on thin materials

The down side is - more set-up cost and more operatortraining.

Pulsed TIG consists of

Peak current - This is set up higher than for non-pulsedTIG.

Background current - This is set lower than peak cur-rent and is the bottom current the pulse will drop to, butmust be enough to keep the arc alive.

Pulses per second - This is the number of times persecond that weld current reaches peak current.

% on time - This is the pulse peak duration as a percent-age of the total time, which controls how long the peakcurrent is on for before dropping to the background cur-rent.

The pulse and base current periods are also controllable.



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When welding is done with pulsing welding mode the weldis in principle a row of spot welds overlapping to a lerger orsmaller extent depending on the welding speed. (Fig.8)

Many double-current machines are equipped with a con-trol function which makes it possible to modify the curveof the alternating current in balance between thew positiveand the negative semi-periods.



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Shielding gasesChemical activity of shielding gases: The behaviour ofgases in welding is related to their chemical activity so itis convenient to group them according to this activity.

Inert gases: These are argon and helium. Other inertgases such as krypton, Radon, xenon and neon have beentried, but their low availability results in them being expen-sive. Also their characteristics do not, at present, givethem any particular advantage.

Argon and helium are monatomic (their molecule containsonly one atom) and do not react with other bodies (in thearc plasma) and hence the designation ‘inert’. This pre-cious property allows them to protect the electrode andmolten metal against the atmospheric gases. Howeverthey are not suitable in every case. Pure argon for exampledoes not allow a smooth droplet transfer when weldingcarbon steels. To obtain the desired transfer mode it isnecessary to add a certain proportion of oxygen or carbondi oxide.

The different ionisation potential of argon and helium causethem to behave differently.

Properties of argon and helium gasThese gases are colourless, odourless.Argon is heavier than air and helium is lighter than air.They do not chemically react with any metals in hot or coldconditions.They give a good shielding action for molten metal from theatmosphere.

Gases for TIG welding of aluminiumArgon gasAn argon cylinder is identified by the peacock blue colourpainted on it.

Quality : Argon gas of welding quality should be used.

The rate of flow of argon should be adequate to obtain aclean weld. This depends on several factors such as typeof parent metal, current used, shape and size of nozzle,type of joint and whether the work is done indoors oroutdoors. Generally a higher rate of flow is required withhigher welding currents, for outside corner joints, edgewelds and work outdoors. Generally flow rates 2 to 7 litresper minute will be found sufficient to weld all thicknesses.

If tungsten inert gas welding has to be done outdoors duringinclement weather, especially during period of high wind,the welding area should be effectively protected. Draughts

tend to break the gas shielding, resulting in porous andoxide contaminated welds.

The penetration profile of argon shielded welds has acharacteristic shape in the form of a finger.Fig.1

Helium: Helium is used mainly in TIG welding and isnormally used with direct current whatever the metal beingwelded (light alloys, copper, etc.)

The main advantages of helium shielding are:

– Increase in welding speeds

– More intense local heating, important with metalswhich are good conductors of heat

– Fig.2 shows the penetration, profile typical of a heliumshielded weld


Argon/Helium gas properties and usesObjectives: At the end of this lesson you shall be able to• state the properties of argon gas• compare the performance characteristics of argon and helium gas for TIG welding• identify an argon gas cylinder and ceramic nozzles• state the uses of argon and helium gas.

Argon gas gives more penetration than helium gas.

Characteristics and comparative performance of ar-gon and helium as shielding gasesArgonLow arc voltage : Results in less heat; thus argon is usedalmost exclusively for manual welding of metals less than1.6mm thick.

Good cleaning action: Preferred for metals with refractoryoxide skins, such as aluminium alloys or ferrous alloyscontaining a high percentage of aluminium.

Easy arc starting: Particularly important in welding of thinmetal.


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Arc stability is greater than with helium

Low gas volume: Being heavier than air, argon providesgood coverage with low gas flows and it is less affected byair drafts than helium

Vertical and overhead welding: Sometimes preferredbecause of better weld puddle control but gives lesscoverage than helium.

Automatic welding: May cause porosity and undercut-ting with welding speeds of more than 60cm per min.Problem varies with different metals and thicknesses andcan be corrected by changing to helium or a mixture ofargon and helium.

Thick work metal: For welding metal thicker than 5mm amixture of argon and helium may be beneficial

Welding dissimilar metals: Argon is normally superior tohelium

HeliumHigh arc voltage: Results in a hotter arc, which is morefavorable for welding thick metal (over 5mm) and metalswith high heat conductivity.

Small heat affected zone:With high heat input andgreater speeds, the heat affected zone can be kept narrow.This results in less distortion and often in higher mechani-cal properties.

High gas volume: Helium being lighter than air, gas flowis normally 1 1/2 to 3 times greater than with argon. Beinglighter, helium is more sensitive to small air drafts, but itgives better coverage for overhead welding and often forvertical position welding.

Automatic welding: With welding speeds of more than60cm per min. welds with less porosity and undercuttingmay be attained (depending on work metal and thickness).

Comparision between argon and helium shielding

Argon Helium1 Smoother arc. 1 Smaller heat affected zone.2 Easy starting. 2 Best for thicker metal welding due to higher arc

voltage.3 Best for thinner metal welding due to lower arc 3 Better for welding at higher speed.

voltage.4 Good cleaning action while welding Al. 4 Gives better coverage in vertical and overhead

positions.5 Heavier than air - Lower flow rates. 5 When used in back shieldings flatens the root

face.6 Lowe cost, more availability.

7 Better for welding dissimilar metals.

8 Better control of puddle on positional joints.

Ceramic shields/nozzles: Gas nozzles are usually de-signed for installation into a particular type of torch andgenerally do not adapt to another make or model. Theycome in all sizes, shapes and materials. Gas nozzles arereasonable in cost, therefore they should be replaced whenthey become unusable. A nozzle which has chips orcracks or a metal build up on the outlet end should bediscarded. These types of defects can alter the gas flowpattern from the nozzle and cause contamination of theweld metal. Typical nozzle configuration are shown inFig.3.

Nozzles are identified by the size of the orifice (opening)and the length of the nozzle as shown in Fig.4 Each torchmanufacturer assigns part numbers to the various nozzlesfor individual type torches and these must be used whenordering for replacement of nozzles.



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Gas lens: The gas lens device allows the welder to use alonger electrode extension than with a standard nozzle.The device uses a series of stainless steel wire meshscreens to ‘firm up’ the argon gas column. This aids inmaintaining a blanket of inert gas around the tungsten andover the weld area. This is very helpful when wind drafts arepresent, or the tungsten must be extended due to thelocation of the weld area. Use of the gas lens requires aspecial gas lens collet body and gas nozzle. A gas lensdevice is shown in Fig. 5 & 6.

Size in mm No.

5 No.36.3 No.48 No.510 No.611 No.712.5 No.814.5 No.916 No.1018 No.1120 No.12

Defects causes and remedyObjectives: At the end of this lesson you shall be able to• state the different type of defects in GTAW• state the causes and remedies of GTAW defects.The following table relates to the cause and prevention ofthe more common defects encountered in welds made bythe TIG welding process. (Fig. 1)

Defect Appearance Cause Remedy

Porosity Pin holes in the weld. Insufficient shielding gas. Satisfactory supply gas. CorrectBore of gas nozzle too small ceramic shield. Remove allarc length too long. Surplus degreasing agents and dry.degreasing agent. Shorten arc length.

Undercut Irregular grooves or channels Incorrect welding technique. Correct current. Correct rodCurrent too high. Incorrect manipulation. Clear weld surface.welding speed. at the toes of the weld.

Lack on fusion.Surface on to which weld is Incorrect current level. Correct current. Use correct rod(side root or deposited has not been melted. Incorrect filler rod manipulation. manipulation. Clean plateinter run) Not always visible. Usually Unclean plates surfaces. surfaces.

detected by bend test or bynon-destructive techniques(e.g.ultrasonic flaw detection).

Lack of Notch or gap at the root of a Incorrect preparation and Use the correct preparation andPenetration weld. set up. Incorrect current level. set up. Correct current. Correct

Welding speed too fast. weld speed.

Inclusions Usually internally and only Oxide inclusions. Inadequate Clean all metal surfaces. Ensuredetected by suitable testing cleaning of parent material a satisfactory supply of shieldingtechniques. Normally oxide before welding. Contamination gas. Excluide draughts.or tungsten inclusions. onsurface of filler rod.

Inadequate protection ofunderside of a weld.Loss ofgas shield.



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Defect Appearance Cause Remedy

Cracking Cracks can occur in the weld The type of crack and therefore Use correct welding procedure.metals and in the parent metal its cause will depend on the Pre-heating and post heat treatm-alongside the weld. They may material being welded. The ent. Use correct preparation Setnot be visible on the surface correct diagnosis of the cause up current. Use correct filler rod.and may only be detected by of a crack frequently calls forthe use of suitable testing expert knowledge.techniques. Always adhere strictly to the

procedure specified when weldingmaterials that are susceptible tocracking.

Always ensure the correct type offiller is used and the correctamount of filler metal is added.



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Friction welding process equipment and applicationObjectives: At the end of this lesson you shall be able to• state the principle of friction welding• explain the method of welding• state the application• state the advantages & limitations.

Friction weldingPrinciple: Friction welding uses friction to create heat tofuse two pieces of metals together. This process is usedmainly in butt welding of large sections round rods, veryheavy tubes and pipes.

Method of welding: No external heat is supplied. One ofthe pieces is made to rotate. The ends of the parts to bejoined are then brought together under a light pressure. Theresulting friction between the stationary and rotating partsdevelops the heat required to form the weld. As the metalsurfaces reach the plastic stage, they are forced togetherunder a much higher pressure. The process produces aclean metal-to-metal welding surface. (Fig 1)

A 1/2” diameter low carbon steel rod with weldingtemperature of 1650ºF can be joined with a contactpressure in the range of 5000 to 10000 pounds/inch whilerotating at approximately 3000 rounds per minute for about5 second. Medium and high alloy steeels require heatingpressure (Contact Pressure) ranging from 10000 to 30000pounds/inch and forging pressure between 15000 to 60000pounds/inch.

ApplicationsMetals that can be welded by the friction welding processinclude Carbon Steel, Steel Alloys. Stainless Steel, Copper,Aluminium and Titanium.

Advantages over metallic arc welding– Itis more suitable process for joining dissimilar metals.

– The process produces a clean metal-to-metal weldingsurface.

– A highly skilled welder is not required.

– A weld with less defects can be obtained.

– No filler rod or electrode is required.

– Power consumption is less.

Limitations– The machine is costly.

– Plates/sections of less thickness/size cannot be welded.

– Welding can be done only inside a factory/shop and notat sites.

– Soft metals and metals with low compressive strengthcannot be welded.

– Only butt joint can be done.

– There is a burr surronding the weld area.


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Laser Beam Welding (LBW)Objectives: At the end of this lesson you shall be able to• state the principle of laser beam welding• explain the application of LBW• explain the advantages of LBW.

ProcessElectrical energy stored in a capacitor bank is dischargedinto a flash lamp. The stimulating light source usually in alinear arc discharge lamp such as Zeon, Argon, or Kryp-ton gas flash lamp. When the flash lamp fires, and then isa powerful burst of light that pumps electrons with thelight emitted (Ruby Rod) to higher than normal energylevels. The light emitted by ruby rod is in pulse and is ofsingle wave length travelling parallel to ruby rod. The mir-rors are provided to reflect the light coming to the ends ofruby rod. So that light may pass back through the rubyrod increasing the energy level of electrons further to emitLaser Beam.

It goes through a focusing device where it is pin pointedon the work piece. Fusion takes place and the weld isaccomplished. There are three basic types of Lasers.

a The solid laser

b The gas laser and,

c The semi-conductor.

The type of Laser depends upon the lasing source. TheSolid Laser some type of crystal such as the Ruby or theSapphire used for its lasing ability.

The gas laser consists of a gas (Carbon Di-oxide, Xenon)or a mixture of gases (90% Helium, 10% Neon) containedin a glass tube with highly polished mirrors at each end.One of the most widely used Gas Laser is CO2 Laser. Theradiant energy density of the CO2 Laser is greater thanthat of the sun.

Equipment and setup (Fig 2)

Laser welding (Fig 1)LASER is the acronym for Light Amplification by Stimu-lated Emission of Radiation. Laser Welding is a methodin which work piece is melted and joined by narrow beamof intense Monochromatic Light. (Laser Beam) When thebeam strikes the job, the heat produced melts and fuseseven the hardest materials.

Fig 2 shows a line diagram of a laser beam weldingequipment/setup. The light or heat energy is put into asingle molecule of a substance (ruby or carbon-di-oxide) tocreate the beam. This single frequency energy of the singlemolecule substance in the form of a beam, when travellingbetween the rear and front mirrors, increases in intensityuntil it passes through the partially reflecting mirrors. Therelease of the laser beam is controlled by the operator/welder.



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Electron beam weldingObjectives: At the end of this lesson you shall be able to• state the principle of electron beam welding• state the welding procedure• explain the application of electron beam welding.IntroductionThe use of the Electron Beam in industry is relatively new.The need for Electron Beam Welding developed for weld-ing costly metals such as Titanium, Molybdenum andTungsten as structural components.

ProcessElectron Beam Welding is an automatic welding processperformed in a vacuum without a shielding gas. Neither anelectrode nor a filler rod is used. In the electron beamprocess, electrons are emitted from the heated filamentcalled the 'Cathode' and then focused in to a beam whichis directed at the welding point. When the beam strikesthe welding point, the kinetic energy of the high speedelectrons is converted in to heat. The heat is great enoughto melt and fuse the metal. The speed of electrons in elec-tron beam welding ranges from 48000 kms /second to192000 kms /second, depending upon the voltage of theunit (Fig 1).

Electron beam welding equipmentsAn Electron Beam Welding machine consists basically ofthe following components:

1 Electron Beam Gun(a. Tungsten Filament b. Anode c. Cathode andd. Focusing Coil)

2 A vacuum chamber3 A transport system

Electron beam gunMost electron beam guns used are the triode type. It con-sists of a) Tungsten Filament b) Anode c) Cathode and d)Focusing Coil. The Tungsten Filament units' electronswhen it is heated in vacuum chamber to 2000°C. Elec-trons carry a negative charge and are repelled by the cath-ode electrode and are made to pass through the centralhole of the anode (+). The electrons are greatly acceler-ated by the potential differences between anode and cath-ode. Then, the beam is focused by a lens. (Electromag-netic Focusing Coil) The purpose of the electron into anarrow beam.

Vacuum chamberThe vacuum chamber is operated in conjunction with pumpsand a pumping system to control the vacuum environment.The size and design of the vacuum chamber used in elec-tron beam welding will depend on the dimensions of theweldment.

Transport systemEach machine is equipped with some form of transfer sys-tem to provide movement for the gun and work. Thesesystems are usually driven by DC motors, which controlposition, Location and Velocities of both the gun and thework.

Description of equipment and welding procedureFig.2 shows an electron beam welding equipment/setup.The machine has a chamber in which a filament is used.

The heat of the laser beam which is of high intensity isconveniently directed towards the joint to be welded bydifferent combination of mirrors. This is possible becausethe laser beam can be reflected like light rays. The laserbeam produced can be either a continuous heat source ora pulsed beam. When the beam contacts the base metalto be welded through a lens the heat is instantaneouslyreleased. The amount of heat applied on the basemetal canbe controlled by controlling the input to the laser beamsource depending on the melting of the base metal beingwelded .

ApplicationsLaser Welding is used in the Space, Aircraft, Electronicsindustries for thinner section metals and dis-similar met-als.

Advantages1 Work piece does not get hot except at one point.2 The heat affected zone is narrow.3 No electrode / filler rod is required.4 Sensitive materials can be welded.Disadvantages1 It has high capital and operating cost.2 It needs a skilled operator.



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This filament emits (gives off) a stream of electrons. Theseelectrons from the emitter passes through an anode,column valve, diaphragm and then passes through anelectromagnet called magnetic lens. This magnetic lens isused to focus the high energetic electron beam which alsopossesses a high heat intensity. The work piece to bewelded is kept in a vacuum chamber below the magneticlens. The electron beam can be directed either downwardsor deflected in any other direction within the vacuumchamber by using deflection coils. The welding is done ina vacuum chamber because the air if present will interferewith the electron beam and affect the welding action. Alsothe vacuum acts as a shield against radiation hazards. Theheat energy required to melt the base metal is controlledby the current in the electron gun filament. The operator/welder can watch the welding area on the joint through asafe optical viewing system containing optical mirrors. Thesurfaces to be joined should be properly cleaned beforewelding. The parts to be joined are to be held together verytightly and moved within the vacuum chamber usingsuitable trollys.

Advantages1 The weld zone and heat affected zone are relatively

small in electron beam welding.2 Distortion affects only a very small area.3 Extremely successful in achieving deep penetration.4 On steel plate 100 mm. thick welds can be made in one

pass at a speed of 170 mm/minute with penetration.5 Usage of current only in the milliampere range is

required, while the other electrical welding systemmany amperes are required.

6 Speed of welding is high.7 No porosity and contaminations.8 Very useful to weld Titanium, Zirconium, Molybdenum

with same control of purity as in the original material.Disadvantages1 The cost of the equipment is high.2 Obstructed joints can't be welded because electron

beam travels in straight line.3 Job is limited in sized as per the work chamber

dimensions.



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Plasma arc weldingObjectives: At the end of this lesson you shall be able to• state the types of plasma arc welding• state the equipments• state the applications.

Plasma Arc Welding is welding process in which plasmaproducing gas (Argon, Nitrogen, Helium, and Hydrogen)is ionized by the heat of an electric arc and passed througha small welding torch orifice. A shielding gas protects theplasma arc from atmospheric contamiantion in welding orcutting. A non-consumable Tungsten electrode is used inPlasma Arc Welding and additional metal is added to theweld with a filler rod.

Plasma Arc welding uses the keyhole method to obtain afull penetration and can be done manually or automati-cally. The works of temperature obtained in this processis about 20000ºC to 30,000ºC.

It is divided in to two basic types. They are:

1 Transferred arc

2 Non-transferred arc

Transferred arc process (Fig 1):The arc is formed be-tween the electrode(-) and the work piece (+). In otherwords, arc is transferred from the electrode to the workpiece. A transferred arc possesses high energy densityand plasma jet velocity. For this reason it is employed tocut and melt metals. Besides carbon steels this proocess

can cut stainless steel and nonferrous metals also whereoxyacetylene torch does not succeed. Transferred arc canalso be used for welding at high arc travel speeds.

Non-transferred arc process (Fig 2)


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The arc is formed between the electrode(-) and the watercooled constricting nozzle(+). Arc plama comes out ofthe nozzle as a flame. The arc is independent of the workpiece and the work piece does not form a part of the elec-trical circuit. Just as an arc flame, it can be moved fromone place to another and can be better controlled. Thenon transferred arc plasma possesses comparatively lessenergy density as compared to a transferred arc plasmaand it is employed for welding and in applications involvingceramics or metal plating (spraying).

Equipments1 DC power source

2 Welding control console (Contain flow meter)

3 Recirculating water cooler

4 Plasma welding torch (up to 500 amps capacity)

5 Gas cylinders and a gas supply

6 Gas pressure regulator

7 Gas hoses and hose connections

8 Water cooled power cables

Gases for plasma welding• Argon for carbon steel, titanium, zirconium, etc

• Hydrogen increase heat Argon + (5-15%) Hydro-gen for stainless steel, Nickel alloys, Copper alloys

Plasma process techniques1 Microplasma

• very low welding currents (0,1-15 Amps)

• very stable needle-like stiff arc & minimises arc wan-der and distortions

• for welding thin materials (down to 0,1 mm thick),wire and mesh sections

2 Medium current plasma• higher welding currents (15-200 Amps)

• similar to TIG but arc is stiffer & deeper penetration

• more control on arc penetration.

Microplasma and medium current plasmaadvantages

• energy concentration is greater & higher weldingspeed

• energy concentration is greater & lower currentisneeded to produce a given weld & less distortions

• improved arc stability• arc column has greater directional stability• narrow bead & less distortions• less need for fixturing• variations in torch stand-off distance have little ef-

fect on bead width or heat concentration & positionalweld is much easy

• tungsten electrode is recessed & no tungstencontami nation, less time for repointing, greater tol-erance to surface contamination (includingcoatings).

Microplasma and medium current plasmalimitations (Fig 3)

• narrow constricted arc & little tolerance for joint mis-alignment

• manual torches are heavy and bulky & difficult tomanipulate

• for consistent quality, constricting nozzle must bewell maintained

3 Keyhole plasma welding (Fig 5)• welding currents over 100 Amps

• for welding thick materials (up to 10 mm)

Keyhole plasma welding advantage• Plasma stream helps remove gases and impuri-

ties.

• Narrow fusion zone reduces transverse residualstresses and distortions.

• Square butt joints are generally used and reducedtime preparation.

• Single pass welds and reduced weld time

Keyhole plasma welding limitations• more process variables and narrow operating

windows

• fit-up is critical

• increased operator skill, particularly on thickermaterials Ù high accuracy for positioning

• except for aluminium alloys, keyhole welding isrestricted to downhand position

• for consistent operation, plasma torch must bewell maintained



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• Medium current: 15 to 200 amps – used as alterna-tive to conventional TIG for improved penetration andgreater tolerance to surface contamination. Generallymechanised due to bulkiness of torch.

• Keyhole plasma: over 100 amps – By increasing cur-rent and plasma gas flow a very powerful beam is pos-sible which can achieve full penetration in 10 mm stain-less steel.During welding the hole progressively cutsthrough the metal with the molten weld pool flowingbehind to form the weld bead.

Limitations of plasma arc welding1 PAW requires relatively expensive and complex equip-

ment as compared to GTAW; proper torch maintenanceis critical

2 Welding procedures tend to be more complex and lesstolerant to variations in fit-up, etc.

Application of the plasma processThree operating modes possible by varying current borediameter and gas flow rate

• Micro plasma: 0.05 to 15 amps – used for weldingthin sheet down to 0.1mm eg SS bellows and wiremesh, welding of surgical instruments, repair of gasturbine engine blades, electronic components and mi-cro-switches etc.



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Fabrication Related Theory for Exercise 2.4.96Welder - Plasma arc cutting & resistance welding

Types of plasma arc, advantages and applicationsObjectives: At the end of this lesson you shall be able to• state the principle of plasma arc cutting• explain the process variable of plasma cutting• state the advantages of plasma cutting.

Cutting processes - plasma arc cuttingPlasma arc cutting process, was introduced in the indus-try in the mid 1950s. The process is used to cut all met-als and non-metals. The common oxy-fuel cutting pro-cess (based on a chemical process) is suitable for cuttingcarbon steel and low alloy steel cutting only. Materialssuch as copper, aluminium and stainless steels were ear-lier separated by sawing, drilling or sometimes by powerflame cutting. These materials are now cut using a plasmatorch, at faster rates and more economically. The Plasmacutting process is basically a thermal cutting process,free of any chemical reaction, that means, without oxida-tion. In plasma arc cutting an extremely high temperatureand high velocity constricted arc is utilized.

Principle of operationPlasma arc cutting is a process resulting from ionizing acolumn of gas (argon, nitrogen, helium, air, hydrogen ortheir mixtures) with extreme heat of an electric arc. Theionized gas along with the arc is forced through a verysmall nozzle orifice, resulting into a plasma stream of highvelocity (speed up to 600 m/sec) and high temperature(up to 20000°K). When this high speed is reached, hightemperature plasma stream and electric arc strike theworkpiece, and ions in the plasma recombine into gasatoms and liberate a great amount of latent heat. Thisheat melts the workpiece, vaporizes part of the materialand the balance is blasted away in the form of moltenmetal through the heat (Fig 1).

Plasma cutting system (Fig 2,3,4)Plasma cutting requires a cutting torch, a control unit, apower supply, one or more cutting gases and a supply ofclean cooling water (in case water-cooled torch is used).


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i Torch design - constricting nozzle shape and size.ii Process variation - dual gas flow, water injection, air

plasma.iii Cutting gas type and its flow rate.iv Distance between nozzle and job.v Cutting speed.vi Plasma cutting current.vii Power used during cutting.viii Manual/machine cutting.ix Material to be cut and its thickness.x Quality of cut required - rough or smooth.xi The bevel angle and round off corner etc.

Advantages of plasma cuttingi All metals and non-metals can be cut due to the high

temperature and high velocity plasma flame.ii Cuts are of very clear form with little or no dross.iii High speed piercing is achieved.iv Cutting of piled plates is possible, even with different

materials.v Cutting cost is quite low as compared to other pro-

cesses, especially for stainless steels.vi Cutting speed is high.vii Cutting is possible in all positions and locations (un-

derwater also).Gases for plasma cutting (Fig.7)• no need to promote oxidation & no preheat• works by melting and blowing and/or vaporisation• “gases : air, Ar, N2, O2, mix of Ar + H2, N2 + H2

• air plasma promotes oxidation and increased speedbut special electrodes need

• shielding gas - optional• applications : stainless steels, aluminium and thin

sheet carbon steel.

Equipment is available for both manual and mechanicalcutting. A basic plasma arc cutting circuit is shown inFig.1. It employs direct current straight polarity (DCEN).The nozzle surrounding the electrode is connected to theworkpiece (positive) through a current limiting reisitor anda pilot arc relay contact.

The pilot arc between the electrode and nozzle is initiatedby a high frequency generator connected between the elec-trode and nozzle. The orifice gas ionized by the pilot arcis blown through the constricting nozzle orifice and formsa low resistance path to ignite the main transferred arcbetween the electrode and the workpiece when the ON/OFF switch is closed. The pilot arc relay may be openedautomatically when the main arc ignites, to avoid unnec-essary heating of the constricting nozzle. The constrict-ing nozzle is of copper and normally water cooled to with-stand the high plasma flame temperature (about 20000°K)and to have longer life.

In conventional gas plasma cutting, discussed above, thecutting gas can be argon, nitrogen, (argon + hydrogen), orcompressed air. For all the cutting gases other than com-pressed air, the non-consumable electrode material is 2%thoriated tungsten. In air plasma cutting (Fig 2) wheredry, clean compressed air is used as the cutting gas, theelectrode of hafnium or zirconium. In used because tung-sten is rapidly eroded in air. Wet and dirty compressedair reduces the useful life of consumable parts and pro-duces poor quality.

Several process variations are used to improve the cutquality for particular applications. Auxiliary shielding inthe form of gas or water is used (Fig 3) to improve the cutquality and to improve the nozzle life. Water injectionplasma cutting (Fig 4) uses a symmetrical impinging wa-ter jet near the constricting nozzle orifice to further con-strict the plasma flame and to increase the nozzle life.Good quality cut with sharp and clear edges with little orno dross is possible in water injection plasma cutting.

Process variables (Fig 5 & 6)





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Principle of resistance welding: Resistance welding isa welding process wherein coalescence is provided by theheat obtained from the resistance offered by the work to theflow of electric current in a circuit and the joint is effectedby the application of pressure.

The fundamental principle on which all resistance weldingis based is as follows.

The heat is generated due to the resistance offered by theparts to the passage of heavy electric current for a fractionof a second.

Heat produced at the junction is calculated by the formula

H = I2Rt

where H for Heat, I stands for the amount of current in amps.

R for resistance offered in ohms

t - time taken for duration of current flow in seconds.

This heat at the junction of the two parts changes the metalto a plastic state, and when combined with the correctamount of pressure, fusion takes place.

The different types of resistance welding machines arespot welding, seam welding, projection welding, flash buttwelding and upset welding machines.

A standard rocker arm type resistance welding machine isshown in Fig.1. The main parts are:

2 Force mechanism: The compressed air cylinder andthe pivoted rocker arm gives the necessary high pressureto the lever to which the upper electrode holder is attached.

3 The electric circuit: It consists of a step down trans-former which provides for the necessary current to flow atthe point of weld.

4 The electrodes: The electrodes include the mechanismfor making and holding contact at the weld area.

5 The timing controls: The switches which regulate thevalue of current, current flow time and contact period timeas the timing controls.

6 Water cooling system to circulate cooling water to theelectrodes.

This is the additional part consisting of a water reservoir andflow system.

Spot welding: This type of resistance welding machine ismost commonly used for resistance welding. The materialto be joined is placed between two electrodes as shown inFig 2a. Pressure is applied after a quick shot of electricityis sent from one electrode through the job to the otherelectrode.

Spot welding is made in three steps.

Fabrication Related Theory for Exercise 2.4.97&98Welder - Plasma arc cutting & resistance welding

Resistance welding process & applications and limitationsObjectives: At the end of this lesson you shall be able to• explain the principle and types of resistance welding process• explain the main elements of a resistance welding machine• state the applications of resistance welding in industry and its advantages.

1 The frame: It is the main body of the machine whichdiffers in size and shape for the stationary and portabletypes.


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The first step is when the parts to be joined are clampedbetween the electrodes. In the second step, a high currentis allowed to pass through the clamped members and israised to the welding temperature. The third step sees thecurrent being cut off and high pressure being applied to thejoint and the joint completed. A nugget is formed as shownin Fig 2b.

A special copper alloy material has been developed for useas electrodes.

Cooling of the electrodes is accomplished by internallycirculating water.

Electrodes are of many shapes and sizes, the mostcommon being the centre tip and offset tip types. (Figs 3and 4)

Spot welds may be made one at a time or several weldsmay be completed at one time.

Spot welding is utilized extensively for welding steel, andwhen equipped with an electronic timer, it can be used forother materials, such as aluminium, copper, stainlesssteel, galvanised metals etc.

Seam welding: Seam welding is like spot welding exceptthat the spots overlap one another, making a continuousweld seam. In this process the metal pieces pass betweenthe roller type electrodes as shown in Fig 5.

Regular spot welding leaves slight depressions on themetal. These depressions are minimized by the use oflarger sized electrode tips and by inserting 1.6 mm coppersheets between the electrode and the job.

As the electrodes revolve, the current is automaticallyturned ‘on’ and ‘off’ at intervals corresponding to the speedat which the parts are set to move. With proper control, itis possible to obtain airtight seams suitable for containers,water heaters, fuel tanks etc.When spots are not overlapped long enough to produce acontinuous weld, the process is sometimes referred to asroller spot welding.Cooling of the electrodes is accomplished either by circu-lating water internally or by an external spray of water overthe electrode rollers.Both lap and butt joints are welded by seam welds. In thecase of butt joints, foils of filler metals are used on thejoints.Projection welding: Projection welding involves the join-ing of parts by a resistance welding process which closelyresembles spot welding. This type of welding is widelyused in attaching fasteners to structural members.

The point where welding is to be done has projectionswhich have been formed by embossing, stamping ormachining. The projections serve to concentrate the



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welding heat at these areas and facilitate fusion without thenecessity of employing a large current. The welding processconsists of placing the projections in contact with themating part and aligning them between the electrodes (flatcopper electrode) as illustrated in Fig 6.

Either single or a multitude of projections can be weldedsimultaneously.

Not all metals can be projection-welded. Brass and copperdo not lend themselves to this method because theprojections usually collapse under pressure. Galvanisediron and tin plates, as well as most other thin gauge steels,can be successfully projection-welded.

Flash butt welding: In the flash butt welding process thetwo pieces of metals to be joined are firmly held in clampswhich conduct current to the work. (Fig 7)

The ends of two metal pieces are moved towards and awayfrom each other until an arc is established. The flashingaction across the gap melts the metal, and as the twomolten ends are forced together, fusion takes place. Thecurrent is cut off just before the heavy pressure is appliedthrough the movable clamp.

Flash butt welding is used to butt-weld plates, bars, rods,tubing and extruded sections. It is not generally recom-mended for welding cast iron, lead and zinc alloys.

The only problem encountered in flash butt welding is theresultant bulge at the point of the weld. It should be removedby grinding or machining if the part needs finishing.

Butt or upset welding (Slow butt weld)

In butt welding the metals to be welded are in contact underpressure. An electric current is passed through them, andthe edges are softened and fused together as illustrated inFig 8.

This process differs from flash butt welding in that constantpressure is applied during the heat process which elimi-nates flashing. The heat generated at the point of contactresults from resistance. The operation and control of thebutt welding process is almost identical to that of flash buttwelding.Butt or upset welding is limited to parts with a cross-section area of not more than 200-250 mm2. Bars withcross-sectional area of 250mm2 and above are joined byflash butt welding.Application: Spot, seam and projection welding is widelyused in the production of cars, tractors, farm machines, railcoaches etc. where thin sheets are to be joined.Large sections like square, rectangular, cylindrical rodswith regular and irregular end faces are welded without anyedge preparations by flash butt or butt welding processes.Advantages of resistance welding– Widely used for joining sheet metals.– Speedy process.– No distortion.– Less skilled operators can do the job.– No problem of edge preparation.



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Fabrication Related Theory for Exercise 2.5.99Welder - Repair and maintenance

Metallizing, types, principles equipments, their advantages and its applicationObjective: At the end of this lesson you shall be able to• explain the purpose of metallizing in various process.

DefinitionMetallizing is a very common coating process which isused to improve the resistance of material agent/againstcorrosion, wear and fatigue.

Metallizing is the general name for the technique of coatingmetal on the surface of objects. Metallic coating may bedecorative, protective or functional.

TypesMetallizing can be done by following

1 By electric arc spray process

2 By spray process

3 By thermal spray coating

Application1 Arc metalizing of products that will not corrective or

rust proof

2 A steel structure protected by metalizing.

3 To improve the resistance of the material against thecorrosion.


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Principles of operations and applicationsObjective: At the end of this lesson you shall be able to• describe the manual powder coating process.

Principles of powder coatingThe powder coating process is very similar to a paintingprocess except that the paint is a dry powder rather thana liquid.

The powder sticks to the parts due to electrostatic chargingof the powder and grounding of the parts.

Any substance can be used that can tolerate of the heatof curing the powder and that can be electrically groundedto enhance charged particle attachment. The powder flowsand curves during the application of heat.

Advantages of powder coating1 Powder recovery for reuse

2 Expenditure will be less

3 Can be more durable than paints

4 Easily can do a work

Disadvantages of powder coating over paints are

1 Can have less leveling than paint

2 Curing is similar, typically more energy intensive thanpoint drying due to higher temperature requirements.

3 Difficult to set a certain plants.

Operations1 Cleaning

2 Rinsing

3 Phosphating

4 Drying

5 Powder coating

6 Curing

Uses of powder coating operating wings1 Railway factory

2 BEML factory

3 Dozer can be painted

4 Complicated parts are to be painted

5 Used in large scale industries

6 Maintenance of fabricated parts.

Fig 2


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Welding codes and standards - WPS & PQRObjectives: At the end of this lesson you shall be able to• describe the welding codes and standards• explain about WPS & PQR

Welding procedure, Performance, Qualification andcodesIntroduction‘Code’ is any set of standards set forth and enforced by alocal government for the protection of public safety, healthetc.. as in the structural safety of building, (building code)health requirements for plumbing, ventilation etc.... (Sanitaryor health code) and the specifications for fire escapes orexits (Fire code)

‘Standard’ is defined as ‘something considered by anauthority or by general consent as a basis of comparison,an approved model’.

As a practical matter, codes tell the user what to do andwhen and under what circumstances to do it. Codes oftenlegal requirements that are adopted by local jurisdictionsthat then enforce their provisions.

Standards tell the user how to do it and are usuallyregarded only as recommendations that do not have theforce of law.

The uses of welding in Engineering Industries are Boilers,Heat Exchangers, Pressure Vessels, Bridges, Ships,Pipelines, Reactors, Storage tanks, Construction Struc-tures and Equipment etc. When a design engineersdesigns a welding structure, the function of production &Quality control personnel is to translate that design in to areal component.

From a design point of view properties of the weld joint aredesigned as

1 Physical soundness (free from discontinuities)

2 Related Theory for Exercise 2.6.06 Metallurgicalcompatibility (Chemistry of weldment, base metal, gasetc.)

3 Mechanical Properties

The welding Procedure Specification (WPS) is writtenexactly to translate these property requirements on torelevant welding variables.

The procedure has to be testified on a test piece for itsintented performance by a qualified welder. To draw acorrect weld procedure, performance methods and qualifi-cation criteria, there are popular codes and standards areavailable.

All the codes specifies the rules for the preparation ofwelding procedures specification and the qualification ofwelding procedures, welders and welding operators. Thiscode specifies the rules for all manual and machine weldingprocesses.

Reading of Welding Procedure specifications (WPS)& Reading of Procedure Qualification Record (PQR)Government as well as private organizations develop andissue standards that apply to a particular area of interest.Many standards with regard to the welding industry areprepared by the American Welding Society (AWS). Manycountries have their own national standards on the sub-ject of welding.

The following are examples of the various standards, andthe bodies responsible for them.

Standard codes Country Responsible bodies

IS India Bureau of Indian Standards (BIS)

BS U.K British Standard issued by British Standard Association

ANSI U.S.A The American National Standards Institute (ANSI)

AWS U.S.A American Welding Society

ASME U.S.A American Society of Mechanical Engineers

API U.S.A American Petroleum Institute

DIN Germany German standard issued by the Deutsches Institute fuer Normung

JIS Japan Japanese industrial standard issued by the Japanese standardsAssociation


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There is also the International Organization forStandardisation (ISO). The main goal of ISO is to estab-lish uniform standards for use in international trade.

The American Welding Society publishes numerous docu-ments on welding and some of them are listed below:

Welding procedure qualificationA welding procedure qualification is the test to prove thatthe properties of a weld to withstand the service condi-tions as designed for particular/specific purpose.

Welder performance qualificationA welder's performance qualification is the test to certify awelder's or a welding operator's ability to deliver consis-tently quality welds. This performance qualification is al-ways done in accordance with a qualified weld procedurespecification.

Weld procedure specificationA WPS is deemed to have been qualified if through teststhat are conducted on the weld test coupon meeting therequirements or the acceptance criteria. Acceptance cri-teria and the specification format may vary depending onthe code of design and manufacture. The tests that arecarried out on the weld test coupon are destructive tests,and they help to evaluate the mechanical properties of theweldment carried out in accordance with WPS.

The results of this qualification are generally recorded in aformat and these are generally recorded in a particularformat and this is usually referred to as an ProcedureQualification Record (PQR). Thus for every WPS therehas to be at least ONE PQR and vice versa.

A performance qualification is generally done to evaluatethe performance of a welder on a welding operator. It isdone to evaluate the ability of a welder or operator to per-form consistently and deliver sound and good quality welds.As this is done to a WPS which has already been quali-fied most codes of practice generally permit the evalua-tion to be done by the use of non destructive tests viz,radiography. Welders and operators who fulfill the require-ments are deemed to be certified for welding to the spe-cific WPS/WPSs.

ASME sections IX, AWS B2.1, API 1104 are some of thepopular American codes specifying welding proceduresand welder performance qualification.

BS 2633, BS 4870/4871, BS 4872, DIN 8560, AD MerkblattHP 2 and HP 3, eN 288-2 and EN 287-1 are some of theEuropean standards for welding procedures and perfor-mance qualification.

IBR chapter 13, IS 2825, IS 7307, IS 7310, IS 7318 arethe major Indian codes on welding qualifications.

Weld procedure specifications, variables and logicfor requalificationA WPS (Weld Procedure Specification) is a documentwhich lists out all the essential characteristics for per-forming a weld. For purposes of qualifying for the WPS, atest coupon is welded adhering to all parameters as stated/

listed in the WPS. A WPS is valid only when supportedby a relevant PQR.

The characteristics listed in the WPS, those in this chap-ter, are otherwise known as variable. As the term signi-fies, these characteristics may be changed or varied.When these "variables" are changed we have a new WPS.Whenever a change in a particular "variable" is bound toinfluence the mechanical properties of the weld, then that"variable" is termed as an ESSENTIAL variable. The vari-able which do not have any impact on the mechanicalproperties of the weld are generally termed as NON-ES-SENTIAL variables. However, under certain conditions,some of the variables could influence the mechanical prop-erties of the weld. Such variables are termed as supple-mentary essential variables. A more detailed treatment ofthese is made in the code of manufacture and the samecould be referred to.

Similarly those variable that have an influence on thewelder's ability to produce sound welds are referred to asessential variables for purposes of Welder PerformanceQualification. An example that comes to one's mind rightway would be the position in which a weld is made.

Introduction to ASME Sec.IXWelding procedure and performance qualificationSection IX of the ASME code specifies the rules for thepreparation of welding procedure specification and thequalification of welding procedures, welders and weldingoperators.

This code specifies the rules for all manual and machinewelding processes.

MaterialsAll the materials that can be used for pressure vesselmanufacture have been grouped (Table 1) under different'P' numbers. The object of grouping the base materials isto reduce the number of qualifications required. The 'P'numbers grouping of materials is based essentially oncomparable metal characteristics such as composition,weldability and mechanical properties.

Table 1'P' Number grouping

P1 to P11 Steel and steel alloy

P21 to P30 Aluminium and aluminium based alloys

P31 to P35 Copper and copper based alloys

P43 to P47 Nickel and nickel based alloys

P51 to P52 Titanium and titanium based alloys.

Filler metalsThe filler metals are grouped as both "F" numbers and "A"numbers.



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"F" numbersAll the electrodes and filler metals are grouped under dif-ferent "F" numbers. The object of the "F" number grouping(Table 2) is to reduce the number of welding proceduresand performance qualifications.

Table 2"F" Number grouping

F1 to F6 Steel and steel alloys

F21 to F24 Aluminium and aluminium based alloys

F31 to F 37 Copper and copper based alloys

F41 to F45 Nickel and nickel based alloys

F51 Titanium and titanium alloys

F61 Zirconium and zirconium alloys

F71 to F72 Hard facing weld metal overlay.

The "F" number grouping is based essentially on theirusability characteristics, with respect to coating. This fun-damentally determines the ability of the welder to makea satisfactory weld with a given filler metal. For example,the low hydrogen electrodes have been grouped under "F"Number 4 and rutile steel electrode4s under "F" Number2.

Obviously, a welder who is able to produce a sound weldwith a E6013 (rutile) electrode may not be able to producea sound weld with a low hydrogen lime powder coatedelectrode.

The skill required to use these electrodes is definitely notthe same. "F" Number 1 is thus the easiest (iron powder)electrode used only in downhand fillet/butt and horizontalfillet positions.

'A' NumbersA part from classifying the filler metals under "F" num-bers, they are again classified under 'A' number as shownin Table 3. 'A' number classification of the filler metals isbased on the weld metal chemical analysis whereas the'F; number classification is based on the usability, or ratheroperation characteristics. With these definitions of 'P' num-bers and 'A' numbers, we shall now see what the codesays regarding welding procedures and welders qualifica-tion.

Table 3

Welding procedures qualificationThe codes stipulate that all the details of the welding pro-cedure should be listed in the 'Welding procedure specifi-cation' (WPS).

Each of these welding procedure specifications shall bequalified by the welding of test coupons, and the mechani-cal testing of the specimens cut from these coupons arerequired by this code. The welding date for these cou-pons and the results of these tests shall be recorded in adocument known as 'procedure qualification record (PQR)'.

A WPS may require the support of more than one PQR,while alternatively, one PQR may support a number ofWPSs. A WPS will be applicable equally for a plate, pipeand tube joints. The WPS should contain the followingnine points in detail.

1 Joints: detailsThe groove design, the type of backing used etc. areto be specified in this. If a change in the type of edgepreparation (Single Vee, Single 'U' or double Vee etc.)is made or if the joint backing is removed, a new WPShas to written but need not be qualified by a test.

2 Base metalsThe base metal (P) number and the thickness rangesfor which the procedure is applicable etc. have to bementioned here. If the range of thickness has to beincreased or a change of base metal from one 'P' num-ber to another 'P' number is required, a new WPS shouldbe prepared and supported by a PQR after due tests.

3 Filler metalsThe details of the electrodes, and filler wires such asthe 'F' number, 'A' number and the type of the fillermetals have to be specified here. The electrodes, fluxcompositions, (basic, rutile, etc.) are also to be men-tioned. A change in 'F' number or 'A' number shall re-quire a new WPS and PQR. A change in the diameterof the electrode also requires a new WPS but need notbe qualified by a test. The addition or deletion of fillermetals requires a new WPS and PQR after re-tests.

4 PositionThe positions in which the welding should be done shallbe mentioned here. The qualification test can be donein any position but still the same procedure is appli-cable to all positions.

5 PreheatingThe preheating temperature, interpass temperature etc.shall be clearly specified. If the preheat is to be de-creased by more than 550C, then a new WPS has tobe prepared and qualified by a test.

'A' number groupingA 1 Mild steelA 2 Carbon - MolybdenumA 3 to A 5 Chrome - MolybdenumA 6 Chrome - MartensiticA 7 Chrome - FerriticA 8 to A 9 Chrome - NickelA 10 Nickel - 4%A 11 Manganese-MolybdenumA12 Nickelchrome-Molybdenum



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6 Post - weld heat treatmentThe temperature and soaking time of the post-weld heattreatment shall be shown here. Any change in thisshall require a new procedure qualification.

7 Electrical characteristicsThe type of current, (AC or DC) polarity, amps andvoltage etc. have to indicated here.

8 GasThe shielding gases flow rate, details of gas purgingetc. will be shown here. Change in gas compositionwill call for re-qualification.

9 TechniqueThe details of the welding techniques string or weavebead, method of initial and interpass cleaning, backgouging, single or multiple passes, root grinding etc.,shall be written here. The test welding can be doneeither in a plate or pipe material and in any position.The maximum thickness for which the procedure isapplicable is generally twice the thickness of the testplate or pipe. The welder who welds the test joint isalso qualified for that procedure but only in that posi-tion in which he welds whereas the procedure is appli-cable to all positions. The results of the tests shall berecorded in the PQR including welding, NDT and me-chanical test results.

Welder's qualificationThe purpose of the welder's qualification is to determinethe ability of the welder to make sound welds.

The welder may be qualified, based on the results of themechanical test (two face bends and two root bend testsor four side bend tests) or by radiographic examination ofa minimum length of 150 mm for a plate or the entire weldfor a pipe. The position of the weld joint has been classi-fied as 1G, 2G, 3G, 4G, 5G and 6G. Table 4 shows thepositions qualifying for other positions.

Table 4Range of positions qualified

Test position Also qualifies1G 1G

2G 1G

3G 1G

4G 1G & 3G

5G 1G & 3G

2G & 5G All positions

6G All positions

For positions 1G and 2G (flat and horizontal) qualificationon a plate shall also qualify the welder in pipes. For allother positions, qualification on a pipe shall qualify for platebut not vice versa.

A qualification in a plate or pipe butt joint shall also qualifythe welder for fillet welding in all plate thickness and pipediameters.



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Reading of assembly drawingObjectives: At the end of this lesson you shall be able to• identified the assemblled jobs.

a bracket bearing

Bush Bush

SlottingCamp shaft

b bracket bearing



Internal bush bearing

Various types of bushes

Ball bearing


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Hard facingObjectives: At the end of this lesson you shall be able to• explain the necessity of hard facing• describe the method of preparation for hard facing• describe the various hard facing alloys• explain the advantages of hard facing.

Necessity of hard facing: This operation consists ofdepositing a layer of harder metal on a softer base metal inorder to provide a surface having special properties such astoughness, hardness and resistance to abrasion, heat andcorrosion.

This is also done to build up worn out areas of a hardenedcomponent due to long and continuous use and make themas good as new with low cost and quickly.

Preparation: Clean the surface of the part to be hardfaced, by grinding, machining, filing, chipping or sandblasting until it is free from dirt, scale etc.

Remove sharp corners which melt easily or get oxidized.

Hard facing alloysDifferent groups of materials used for hard facing are:

– Ferrous alloy group– Non-ferrous alloy group– Diamond substitute group

Ferrous alloy group: This group comprises weldingelectrodes having an iron base alloyed with chromium,manganese, molybdenum, nickel, zirconium, boron andsilicon.

Non-ferrous alloy group: This group consists of weldingelectrodes which are alloys of chromium, tungsten, cobaltand molybdenum and some times small quantities of iron.

Diamond substitute group: This group composed ofcarbides of tungsten, tantalum, titanium and boron and theborides of chromium is so called because its hard facingmaterials approach the hardness of a diamond.

Hard facing electrodes are designed on the basis ofhardness of their weld deposits as follows. (table)

Typical deposit composition for high temperature serviceDeposit composition %

Type C Mn Si Cr Ti Mo W Co V NiA 6.0 2.7 1.0 13.0 5.2 - - - - -B 4.6 1.0 1.1 27.0 - 3.5 - - - -C 3.0 1.3 0.81 8.3 1.5 - - - - -D 2.0 - - 29.0 - - 12.0 51 - -E 1.6 1.8 0.9 5.8 4.2 0.5 - - 0.4 -F 0.7 - - 27.0 - - 4.5 62 - -G 1.75 1.0 1.5 30.0 - 1.5 - - - 3H 0.8 0.5 0.65 4.0 - 8.0 1.2 - 1.0 -I 1.9 1.5 0.60 7.4 4.6 1.4 - - - -J 4.5 0.75 0.80 28.0 - - - - - -K 4.2 1.7 0.10 5.0 6.8 - - - - -L 3.1 3.1 1.7 15.0 - - - - - -

NOTE: A to H are manual electrodes; I to L are flux-cored wires.

Type Hardness BHNA 250-280B 350-380C 280-320D 600-625

Electrodes commercially available:

Indian Oxygen: Stellite Grade 1, Stellite Grade 6, StelliteGrade 12, Duriod 1, 2, 3.

Larsen and Toubro LtdEutectrode - 2 (AC-DC)

Advani Orlikon - Citorail I to IV Citomangan, Supersistetc.


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Application: Chromium and tungsten carbide electrodesare used for severe abrasion - resistance.

High carbon type electrodes are used for moderate abra-sion and impact resistance.

Stainless steel electrodes are used for severe impact andmoderately severe abrasion resistance.

Hard facing with MMAW process: Clean the surfacethoroughly and arrange the work in a flat position.

Preheat to about 95°-150°C.

Use only enough amperage to provide sufficient heat tomaintain the arc. Avoid high current and short arc length.

This is very important to prevent dilution of thedeposit with the base metal.

Use the stringer or slight weaving technique holding amedium arc.

Deposit 25 to 50 mm long beads not wider than twice thediameter of the electrode.

Allow the work to cool between each deposit of beads.

Stagger the deposits to prevent building up of high heat atonly one spot.

Chip the slag between passes.

Slow cooling by covering the job with sand or ashes orslaked lime is to be done.

The number of layers will vary from job to job. But it shouldbe noted that the first layer deposited on mild steel isdiluted by the ‘pick-up’ from the plate. (i.e. the soft mildsteel from base metal will mix with the hard depositedmetal and therefore the I layer will have less hardness.

It is never advisable to make more than three layersbecause such a mass of metal may crack in service orduring deposition.

Advantages of hard facingLonger life of wearing parts (2 to 20 times, depending onthe type of service).

Increased mechanical operating efficiency.

Reduced idle time of plant.

Use of reconditioned worn out parts instead of costly newreplacement parts.

Reduced labour costs because of fewer replacements.

Greater independence during periods of replacement ofparts when there is a shortage.

ApplicationsDifferent hard-faced products are illustrated in Figs 1 to 9.





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Fabrication Related Theory for Exercise 2.5.104 & 105Welder - Repair and maintenance

Surfacing/Metal build upObjectives: At the end of this lesson you shall be able to• explain the purpose of surfacing/metal build up• describe the different applications of surfacing• describe the factors that cause wear• describe the method of surfacing by the M.M.A.W. process• state the different surfacing methods.

Purpose of surfacing/metal build up: Surfacing/metalbuild up is done to build up worn out parts whose dimen-sions have been reduced and make them as good as newand obtain the required properties.

ApplicationTo repair worn out shafts, gears and other parts, (Fig 1)surfacing might be the only way to solve the problem sincenew parts may not be available any longer. Surfacing isdone to modify a part’s surface so as to improve itsperformance.

Causes of wearThere are six different factors that cause wear or affect theparts.

Abrasion is the grinding or scratching action of hardparticles that cause wear.

Erosion is the wear caused by liquids or gases strikingmetal parts at high speed.

Impact is a blow or series of blows that can cause the metalto deform, fracture or peel off in pieces.

Corrosion is a chemical or electrochemical attack on asurface.

Rust is caused by oxidation.

Heat softens the metal which reduces the metal’s resist-ance to abrasion and corrosion.

Friction on the surface of the metal results from the metalsrubbing together under pressure with little or no lubrication.

Method of surfacing / Metal build upPadding: A plate of 10 mm mild steel about 150 mmsquare is used for padding. A series of parallel beads arelaid side by side across the surface of the plate so as toslightly overlap one another. If the beads are laid side byside with no overlap, the slag becomes entrapped in the linewhere the beads meet, making it difficult to be removed andcausing blow holes. (Fig 3)

Weld beads running across the surface of a rollercan improve its gripping or driving action. (Fig 2)


110

Each bead is de–slagged before the next is laid.

After thoroughly cleaning and brushing all the slag andimpurities from the layer, another layer is deposited on thetop of this with the beads at right angles to those of the firstlayer. (Fig 4)

This can be continued for several layers and the padfinished.

Building up shafts: In the case of shafts worn out withsmall wear, such as 1 mm, it is evident that if the shafts arebuilt up and then turned down again, the thickness of thedeposited metal remaining is very small, and the bearingsurface will be practically on the line of fusion.

It is advisable to turn the shaft further by 1 mm.

Bring the bearing surface well into the deposited metal.This will give much better service. (Fig 5)

Welding depositionDeposit beads along the side parallel to the axis.

Use a medium arc with stringer beads.

Deposit the run as shown in Fig 6 on the opposite sides,1, 2, 3 and 4 to equalise the stresses. Complete the jobdepositing symmetrically.

Surfacing methods– Oxy-acetylene method

– Shielded metal arc method

– Submerged arc method

– MIG welding method

– TIG welding method

– Plasma arc welding method

Oxy-acetylene welding methodOxy-acetylene method has great usefulness for smooth,precise and extremely high quality surfacing deposits.

Advantages

Very thin layers of weld metal can be applied. Can be easilymade to flow to the corners and edges of the job. Thismethod is smooth, precise and is of extremely high quality.

Grooves can be filled. But this is very slow process.

Shielded metal arc weldingThe equipment required for this method is the same asused for manual metal arc welding.



111

In plant training/project workObjective: At the end of this lesson you shall be able to• prepare utility jobs as per drawings given.

Project work for students

1 Fabrication for metal rack (as per drawing)

2 Fabrication of cylinder trolley (as per drawing)

3 Fabrication of welding fixture (as per drawing)

Students are expected to write following details in brief andfabricate all above jobs.

a Project name

b Material specification and thickness

c Name of the part (s)

d Job sequence

e Skill sequence

f Time availed for completion

g Inspection

• Visual• Dimentions• Fillet sizes etc.

h Method of controlling distortion

k Remarks/assessment

The instructor to guide the further project workfor training purpose

Cylinder trolley with chain provision for locking

Single

Welding fixture



112 Fabrication: Welder (NSQF - 4) - Related Theory for Exercise : 2.5.104 & 105


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WELDER - Bharat Skills · The material is not the purpose of self learning and should be considered as supplementary to class room instruction. TRADE PRACTICAL The trade practical